I'm not going to really offer everything I think needs changing. But I think buffing Rumia's mag could result in some really op moonlight rays. I think upgrading her defenses would make her more attractive without making her op.
Sakuya is weak, but buffing her attack can result in the above like rumia. Her problem is her formulas have terrible atk - enemy def ratios. But the multiplier afterwards is high (killing doll I mean). which means that should you buff her attack to make killing doll a viable boss dpser, on targets with low def, she's going to rival Flan's starbrow break, or if you're simply over level.
In general I think there should be more options for tanks. China and
are your only "real" choices for first slot tanks. Other characters can do it sometimes, but it's going to either be simply shooting yourself in the foot to do so, or it's an exceptional case like a boss who has no physical attacks, or one with defense ignoring attacks.
Chen is a pretty good character overall, despite her evasion being missing, I think buffing her would make her pretty op, which some people feel she is already.
I think most of the balance issues of the game lie in the formulas moreso than the base stats >=(. Aside from the tank issue that is.
Though...
being able to modify the formulas might not be an issue now that I think of it (dun dun dun)... to be continued....