There is no problem with introducing something later into a game, Thousands of videogames wait for the boss fight to present a new combat option. I like the idea of combined attacks providing a multiplier for everyone involved, so the more combos the higher the bonus.
Just as long as they're all legal moves which add to the final effect. Someone just standing on the side lines cheering the squad on wouldn't add to the combo multiplier, but someone on the sidelines playing a rousing anthem to rouse their fighting spirit or summoning demons to drive the fighters onward would.
Basically each player in the chain would have to be using an activated ability or trait, rather than just a standard action in order to benefit from the bonus. Although you could have the final member of the chain using a standard attack after having been buffed by several other players.
Also some powers could take advantage of the status effects applied by other characters, similar to how Deimos can chain
Kai Lao into
Yamazuki into
Kuchiki Daoshi for added effect. For instance, after Bumblebee
Shocks a target Sigma could draw the energy from them, gaining both a boost to the attack and an Energy bonus for the power he's collecting, but negating the
Shocked status effect in the process.
