Just going to throw in my wall of text:
Joe Kucan, Nicolas Worth, Kari Wuhrer, Udo Kier and Michael Biehn. These are only a handful of actors that formed the roster of memorable characters that have featured in C&C over the years. Even though the cost of re-recording such performances with new actors would deem it unlikely, it doesn't need to be done.
An up-scaling of the original footage or re-mastering from the original video reels (if EA or Brett Sperry/Joe Bostic/Louis Castle still has them) would be incredibly welcome. A lot of C&C's memorable moments and quotes come from the cut-scenes, so shipping a remaster without or with new cut scenes would be a great loss indeed.
EVA and Unit V/O
Part of the character of C&C is definitely the V/O. To this day, I can quote C&C units to my friends and they know exactly what I'm referencing. From the Workers complaining about not having shoes in Generals to the Desolator's heavy breathing, to the Nod Harvester in C&C3's Tiberium obsession; the V/O is one of the most memorable and iconic things about C&C and it really helps to define the character of each unit to the point where the combined characters shape the overall character of the game.. Re-recording it just wouldn't be the same.
One thing that has definitely been synonymous with the earlier C&C games is Frank Klepacki's music (and to a lesser extent the jukebox function). The ability to play Hell March on repeat whenever you like during Red Alert 1 skirmishes always brings with it a rush of pure nostalgia. Bringing in Frank to score some new tracks for the remaster would be one of the best things to win the hearts of the fans, as there is a lot of love for Frank and his work within the community. The games that he scored over the years have a certain atmosphere because of the music and it wouldn't be right to bring the remaster out without it.
Modding has always been the backbone of the C&C community, whether it be maps or fully blown total conversion mods. C&C mods consistently make ModDB's top 100 every year and bring more aspiring developers into the industry over time. Modding has taken a bit of a hit recently due to less developers supporting it, along with the rise of proprietary game engines like Unreal and Unity, but despite that, modders still continue to work diligently on mods for older titles and do get recognition for it. The Mental Omega and Twisted Insurrection mods for Red Alert 2 and Tiberian Sun respectively have garnered coverage in PC Gamer this year and continue to provide replay-ability to their parent games long after they came out. If the remasters are to begin their own legacy, modding is a great thing to plan for in order to keep the community coming back for more.
Gameplay, Balance & UI
Classic Mode vs Modern Re-balance
When undertaking a remaster, especially for an older title, players will have certain expectations of how it will ultimately play. C&C was always good in offering a number of different ways to play by a number of toggleable skirmish options. It would be great to see a classic balance option as one of these game options, because it will allow players to play the game exactly as it was when it was released. Playing without this option enabled, would be a more balanced experience overall than, Red Alert 2: Yuri's Revenge for example. To this day, players name their online games with titles like "1v1 No Rush No Yuri", as Yuri is perceived to be the best faction in the game. With Yuri's Revenge only seeing a single patch after release, there were simply not enough balance iterations to fix this problem.
This also allows more agency to the development team to inject more modern RTS features (such as concurrent construction from multiple factories for example) into the game in order to provide a vision that brings the gameplay up to modern standards. A modular ruleset would allow players the highest degrees of flexibility, with the aforementioned factory changes being one, along with an option to use a more modern sidebar, turn units/structures on and off rather than through un-descriptive tech levels, a no-rush timer that limits their unit movement to their section of the map only until the timer expires, multi-engineer (RA, TS) option, fog of war settings (on, off, revealed), diplomacy (mid-match team changing like in RA2), MCV re-packs and the ability to toggle any new features added into the game so that they can be turned off to provide the 100% vanilla experience. A faction banning system that allows the host to ban the selection of certain factions in the lobby would be useful as well (especially in the "No Yuri" example).
Art of Defence
The addition of "Art of Defence" as an official mode in multiplayer and skirmish would be a welcome addition to any C&C remaster. Art of Defence is a wave based defence mode where players have to protect an objective or simply survive until they clear all of the waves of enemy units being thrown at their base. These types of maps are popular with the community and have been produced by modders en-mass. With wave-based survival RTS "They Are Billions" proving to be a popular and refreshing approach to RTS gametypes, I feel an official C&C interpretation of the survival/AoD gametype would prove popular with the fans.
Something that has always seemed like it would be a great quality of life improvement to the classic C&C formula is the ability to queue buildings. OpenRA made this a reality and it does help to take a bit of pressure off the macro-management of base construction when the player needs to be present elsewhere on the map.
Control Groups, Factory Groups and Factory Stances
The ability to bind units into multiple control groups is something that I find really useful in newer RTS titles. The level of control and micro you can potentially setup on the keyboard's number-line is incredibly powerful if used well. The ability to bind buildings to a control group is also incredibly useful in other RTS titles and I think it would be a welcome addition to C&C. There is an understandable wrinkle in the fact that classic C&C games don't allow for more than one factory of a given type to be used, but games which use that system (like 8-Bit Armies for example) seem to have solved it to a point by auto-switching to the appropriate UI tab when the corresponding factory is selected.
An extra function which would make control groups even more powerful would be if you could assign a factory to build units into a particular control group, so that there's no need to keep adding new units into a given control group. C&C3 had a similar feature where the player could set the stance of a factory (Aggressive, Normal, Hold Ground, Hold Fire) which was incredibly useful, for example, if you wanted to build an army that would attack everything straight out of the box (I usually used it in conjunction with Black Hand Flame Tanks with the Purifying Flame upgrade in Kane's Wrath which was incredibly satisfying).
Waypoint Mode & Planning Mode
Waypoint Mode & Planning Mode were both great features in C&C3 that enabled players to have absolute control over their units by making them patrol areas, or execute plans whilst the player focused on other matters. This helped players to plan their micromanagement ahead of time, which could be beneficial in the right circumstances. The differences between the two were subtle, but enough to warrant evaluating them both for inclusion in the remaster.
RMB Scroll Coasting
RMB Scroll coasting (when the player holds the right mouse button and can then quickly move around the map by moving the cursor to the edges of the screen) was an excellent feature in Red Alert 2 and C&C3 that really aided the accessibility of the game. Whilst most contemporary RTS titles compromise the layout of the keyboard to offer WASD movement, or used the frankly quite clunky pull-and-grab camera control method, C&C has always been miles ahead of the camera control game just because scroll coasting is so intuitive.
To echo what others have said previously about the C&C3 sidebar, I'd definitely add my +1 into the ring of supporters. C&C3's sidebar was probably the best iteration of the sidebar to feature in the franchise because of it's unobtrusive design and easy-to-understand tab system. The fact that it reacted to what building the player had selected at the time is just the icing on the cake. Whilst other RTS games seem to clamour for the bottom-bar, the sidebar will forever be a C&C characteristic, though that's not to say that some degree of placement customisation wouldn't be welcome.
Lining up your units into fromation and having them move across the battlefield at the speed of the slowest unit was great for ensuring that your faster scout units didn't run ahead of the player's main force and get themselves destroyed. Also being able to control a unit's facing was really important for deployable units like the GDI Rig, which was stronger from certain sides based upon which was it was facing. Bringing back formations and formation move would really help in the 3D titles, but I'm not sure how much use it would be in the 2D titles, given that units operate on a grid to begin with.
There's not really that much to say here. Up-scaling the artwork in the older pre-Generals titles would be absolutely amazing. Microsoft have done this with Age of Empires: Definitive Edition along with adding more rotation directions for every unit (up to 32 from 8 if I remember correctly), which would be awesome to see in a remaster. That being said, I don't think it would be appropriate to take one of the older games and put it into 3D. It would seem less like a remaster and more like a remake at that point.
Fully Customisable Control Scheme
One of the learnings that I've made over the years is that RTS gamers enjoy customising their controls and hotkeys, but only if the menu to do so is functional. The ability to easily see a list of mapped controls, rebind keys and save/load control configurations would probably appeal a lot to the hardcore RTS scene and empower them to find a control scheme that works for them. Fans of the later titles and other RTS games entirely could reproduce the control schemes that they use into the older titles and share them across the community, which would help to appease players from all walks of RTS life.
Left Click vs Right Click
When C&C3 patched in left click mouse controls, it was incredibly welcomed by the community. I understand this a bit better now by taking a look at my own experiences designing RTS games and watching newcomers to the genre play for the first time at several high-profile gaming events; observing that players that are new to RTS will instinctively attempt to use Left Click for all affirmative actions (selecting units, moving units) until they are explicitly told how the "classic RTS" Left-to-Select/Right-to-Move system works.
In our day to day lives using Windows as an operating system, Left Click is used for all affirmative actions such as selecting, dragging, choosing menu items etc. whereas Right Click is more contextual. Windows trains it's users to expect mouse based interfaces to be this way, so when it comes to RTS that control expectation doesn't necessarily carry over, unless you are playing a classic C&C game where every affirmative action is on Left Click.
With this in mind, I'd highly recommend keeping Left Click orders as the default mouse control method simply because it is a lot more accessible for new players to grasp. An option for right click orders is definitely needed to appease the RTS crowd who expect that, but if C&C is to be brought to a new generation through remasters, I feel strongly that this is one of the small things that could be kept as-is in order to retain the accessibility of the game and keep it easier to play for gamers that are new to the RTS genre. Just because all of the other RTS's have Right Click as default, it doesn't make it the right choice for C&C.
Double Click to Select All Units of a Type
Introduced in Generals, this feature made it really easy to pick out units of a specific type from a group. whilst it's kind of obvious looking at other RTS titles these days (because they all do it), it's worth mentioning regardless.
Shift Click to Queue 5
One of the best control/UI features from Tiberium Wars was the ability to Shift+Click to queue 5 units. This would be greatly appreciated as a quality of life feature when remastering the older titles.
I think that's about it for now!