[@pkken] Hmm... I don't think a hard-and-fast system of numerical stats would work very well, since this wouldn't be dice or stat based. However, on the other hand, giving a general "rating" for individual parameters might be a convenient frame of reference. For example, character A is light and fast, with a focus on agility. Character B, on the other hand, is slow but hard-hitting. So, to describe that, a convenient shorthand might be to have "stats" like "speed" vs "strength," with simple letter rankings. So, Character A would have an A or B in Speed and a C or a D in Strength, while Character B would be the opposite. So, when they're in battle, that allows their writers to look at those and clearly see who's faster, who's stronger, and all that. Those could also be raised somewhat through training and in-character progress, providing a track of how a character has improved over the course of the RP.

Aside from that, though, I'd mostly recommend limiting character progress to just finding new applications to Quirks, surpassing prior limitations, etc.