Different body sizes would work nicely for this game. Small, medium, and large essentially. With that, more equipment types to find, which should spur the player market as players will pick up weapons they can't equip due to size limitations.
Type summary:
Small -
Small characters have a slightly higher run speed, but lower health. Their weapons are on the weak side, but have significantly faster recharge/reload times. They're more towards deaths of a thousand cuts, think of claws, needle guns. Mini-Ninjas.
Medium -
Medium characters are essentially what we have now, a versatile body type.
Large -
Large characters have a slightly slower run speed with higher health. Their weapons are on the powerful side, but take much longer to recharge/reload. It's a focus on killing the opposition before it reaches them, or dealing powerful single shot blows to their opponents. Huge swords and man-portable cannons!
Note that I didn't mention armor, armor should be accessible by all body sizes, just each with a different model. Small sized characters feature more cloth like armor, large characters feature more metal.
In a way, we already have these things: some weapons and armors make you faster, while others slow you down. What is different from your idea is that we don't have to reset our characters if we want to change how we play... instead, we just need to equip some different weapons.
What new benefits would come from changing to your idea?