>>715378124 (OP)One idea I had once was reworking Assist Trophies into a full on "Character Calling" mechanic like in other fighting games. You select an assist character before a match, press a button, assist is summoned, does an attack (or really any other supporting affect) and then disappears with a cooldown until you can summon them again. Simple as that. Would freshen up and substantially change the meta to make it stand apart from the other games, and you could toggle assists off if you just wanted to play normally.
I'd also give the game a flashy comic-inspired artstyle. All of the characters would be stylized to have thick black outlines and more jagged shapes. Something similar to Mario Strikers' artstyle is sort of what I'm imagining.
Other random misc stuff I'd do is rework most of the older veterans movesets to be more modern and diverse, and have an updated Smash Run with more maps and online.