You can check everything DS related in that topic I posted for references - it includes all character, item and action sheets, google docs, locations and media.
Dead Space 1 & 2 was basically just a Dead Space themed Mafia game with a unique mechanic that allowed players to return to the game as Necromorphs when they were killed (not spaced); a unique item Marker (or marker shard) that also changed and/or influenced the characters interacting with it; and finally, similar to Star Wars Mafia or other RPG games I hosted, a unique character development and customization, including weapons and loadouts.
Since DS1 & 2 were done and played on each single ship infested by necromophs there wasn't much of a exploration to do, but in DS3 I also introduced multiple locations, sections, levels and objectives, including more factions with different objectives (because of this we had plenty of of "fun" as cat5 put it).
DS4 was made completely for RPG experience, it was played as D&D, both coop and competitive. Exploration was key to progress in the game, and players had to work together in order to advance to next locations. Each location was a maze of rooms or connections to other locations:
Overall we had 135 playable locations filled with items, puzzles, NPCs/monsters that you had to get through, a couple of minigames, 30 playable characters, 30 different NPCs, 40 different weapons made of several parts that you could craft, ... all packed up and divided into 19 chapters! When I'll have time I'll include more info on this in the topic above since there was just too much of it, and also because there might be a sequel to the RPG to conclude the story (we played the RPG for several months and it was already too much).
I've also made a Battle mode where all the fighting players changed turns in order to fight the NPCs or other players, just like you saw in the Star Wars Duels but with more items, player conditions and actions (similar to Final Fantasy RPG battle system).
First you need to decide how much RPG you want to have - from DS1 to DS4 there's a big difference already. One is done in few weeks other in few months, so you need to plan character development, story, items and overall objectives accordingly to fit in your vision. Once you are decided on that you can start tweaking it as you wish.
In all DS games, player characters were "recycled", which gave the game unique feeling of development. Hence dead players returned as Necromorphs, spaced players returned as Unitologists, or any other faction that was present. It give both the dead and surviving players something to do until the game was concluded (usually everybody died anyway eventually, sometimes we had 1 survivor escaping just like in original Dead Space saga, e.g. the "Alien Ending").
Players usually started as marines and engineers, with only few that had different objective from other TP faction so that we could use traditional mafia mechanics as well. The game developed once people actually started dying, as it introduced more enemies to the surviving factions, but through game progression their characters were already prepared and developed to fight them (well someone was someone wasn't ).
Make an interest threat once there is vacuum of games and see. You really need players to be less occupied for RPG game, trust me on that one.