Yeah, Mewtwo's the only "required" veteran who needs to return from Melee in my mind.
Pichu, Roy, and Young Link are all pretty "eh" to me. Maybe if Toon Link fades (as in he doesn't get any new games) and we get OOT 3D and MM 3D with Young Link, he could come back in Smash 5, but I think Toon Link is pretty popular in his own right now and has Wind Waker (HD), Minish Cap, Phantom Hourglass, Spirit Tracks, and Four Swords Adventure under his belt.
Pichu Bros. seems fun, but that's sure to piss off a lot of people who get really anal about anything inspired by the anime getting into Smash. Apparently there's a huge deal with Greninja using an anime voice because it implies it's meant to be Ash's Froakie, or something, and that actually bothers folks. I don't see the big deal, personally, the anime is a big part of the Pokemon franchise and is still going strong in Japan, I don't have an issue with them drawing from it on a basic level.