That would actually be interesting, though would have quite a big impact, but certainly could alleviate some of the concerns @GaryOaktalked about too, of allied having basically comparable firepower to soviet, while being able to repair in field. If mechs can't drive tanks, well allied would first have to sacrifice at least one person for mech duty, and probably have at least one other vehicle he can be a passenger in to protect him when needed (apc or ranger), which would cut down on the amount of MBT's they have if they want to sustain of the mech, and in really big games/pushes, may need more than one mech to adequately do it. AND since there would have to be used other infantry for the tank driver (probably would be RS and captains mainly), it might be good to bring a medic along too, to heal them, and the mechs up if/when needed, when some of the tanks do go down. So in short it would mean much more varied units would have to be in use in an allied push
Also would mean that phases would have to be a LOT more carefull too, when acting alone, as they can't have a mech inside them either, so taking even a bit a damage, they would pretty much have to retreat to find a mech or an SD, as even a small amount of damage would put them in risk of getting killed by the hard hitters in a single shot.
That said though, yeah I think it could do a lot of good in high player count games, but I do fear it would absolutely decimate allied in low player count games, so unless it is possible to have it so mechs can only be drivers when there is less than say 4 players on the team or something, I fear it would do more harm than good
Other than that yeah the flamethrower and volkov could need some tweaking. Flamethrowers I just end up rarely even getting. And if I get a volkov against inf I end up often just using his AT cannon even against inf, his AP don't seem to do much for me, essentially making him a beefier but very expensive RPG-soldier. Not sure what those tweaks should be exactly though