Yeas, increased EXP rate for tanks would be nice. Especially since your commander outlevels them so fast, and several of the tiles are scaled to your commander, not your tanks LVL.
Increasing all EXP gain from battles by 15-20% should be enough for the tanks to be somehow able to keep up with the commander.
I mean - My Super Pershing Sarah, which I had day one since launch is around L35. My commander is close to hitting L48... yeah, thats more than 10 discrepency, which is way to much in the resulting power difference between you and the enemies.
Never mind the new tanks I swapped in, who are sitting around L20-28, they get mauled bad.
With the exp Increase, all of my tanks would be around 3-5 levels higher by this point, and them being 5* with 5* stat increases, that would actually make a diffrence.
The lack of any EXP items for off battle leveling is also a thing. Past a certain point the EXP gained from using other tanks to upgrade becomes very diminutive, and its just not fun, farming 30 copies of Rene, to lvl her up, rince and repeat 9 more times and then upgrade your desired tank with those copies. That sort of grind is no fun at all considering there is no real rewards at the end.