ooo, remember that kinda white wave effect that spreads out behind Bowser when he performs the sliding haymaker? that could add somethin to it!
also, a plausible idea for making Bowser's fire not damage him when breathing it (from an above message) could be like a timer coded on the fire itself in which it doesn't affect Bowser while active, but once the fire's out far enough the timer would run out and the fire could cause Bowser harm from there, which I think'd work fairly well with pvp or coop, or maybe something similar could be done? Might need to be a flag of its own if done that way though, otherwise it'd affect fire from hazards and opponent attacks too
Either that or could just have it have its own flag and apply that flag to Bowsers so his own fire doesn't harm him.
But ye, as mentioned earlier, the fire was the thing that stuck out as not being there most, the second being Spikeball, which I think'd be pretty cool and fairly useful given Bowser's lack of speed. Just some ideas that could help if needed. Other than that, I absolutely love this to bits, very well done!
Sorry if this seems too long, apparently I can't really do short XD