Different Body Sizes
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.

most people actually like this game because it dosnt have limitations and you can change your style if you don't like it.
and there alread are weapons like these without needeing the limitations
swords (large=Khorovod. medium= spur. small= cutter)
guns (magnus, alchemers, autoguns)
bombs (Blast, element, crystal)
dont change the things most people like.
Not quite, I'm preferring going to the greater extremes of the weapons. Large characters wield powerful cannons and swords which are towards power, rather than elements and debuffs. Smaller characters wield weak weapons, but are much more inclined towards elements and debuffing mobs. The characters we have now are the midway, jack of all trades, master of none. I'm not changing the game, merely adding to it.
I'm intention is to play as a hulking character that runs around with a large rotary cannon!

It'd probably be more preferable to just make that a new weapon type instead of requiring you to be a certain body size. Being a jack of all trades means we have the freedom to do whatever we feel like which is a pretty important feature and tends to be likeable in a lot of games. People tend not to like being restricted and creating multiple characters would be a huge pain. (I don't even know why we have 3 character slots =/) You could have it be an effect of wearing certain armours maybe? That'd fit more into the game's style.
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?