I feel like Sakurai has neglected the Sonic franchise a bit

Yeah, he's missed out on a lot of these characters' abilities in their games like Sonic's boost and Ganon's float ability.

That's true. I was thinking more in terms of the number of playable characters. Half of the legend of Zelda reps in smash are just different Links. There are so many characters that could have interesting movesets from that series that it feels a bit wasted. and then Sonic has so many different characters with fantastic moveset potential, that it feels like a waste that Sonic is the only one there. Even if they were wanting to pad out the roster with more echo fighters, Metal Sonic and Shadow could potentially fill those roles, though independent movesets would make more sense for them.

Could also be due to.copyright and sega

Nintendo and Sega are on better terms than ever before. I don't think It would be much of an issue.

I mean, Xenoblade already has two fighters, and I feel like sonic as a franchise has had a large enough effect on the gaming world as a whole that It deserves At least one more character on the roster. Plus, the Sonic cast is far more recognizable than most of the franchises you mentioned.