The STuG is a great vehicle, looks really cool, and effective too.
A tank railgun would be taking the kinetic penetrator idea to the extreme I think, I don't know the impact on the tank's ability to fight in urban or confined environments though, I feel like firing a railgun at a building up the street could have unintended effects. Tanks suffer in close quarters terrain as it is, having essentially a sniper weapon could be an issue, they may need to have some beefy coaxial weaponry to be multirole. Perhaps a missile launcher combined turret?
Grimmas, that tank is infuriating in tier 1 in War Thunder- early in the game it is difficult to one-shot a tank, and that thing has a silly number of crew making it really resilient to picking them off. Most German tanks can have the crew taken out in 2-3 shots.