Wolf and Ice Climbers are my two most wanted Veterans to return, and I also feel that they both deserve to make a return as well. Pichu and Young Link I'm not too keen on, personally. I could tolerate Pichu, but I feel that other Pokemon are much more deserving to be in Smash for Switch. Young Link I feel should be an alternate for Toon Link if possible, because we don't really need three Links in the game. One is enough for me, personally.
Since we're discussing veterans, might as well post my opinions of the ones not mentioned by OP. Squirtle and Ivysaur I kinda count as a two-in-one sort of thing. Having Charizard and one of the two, but not the other would be strange. If the two came together, that'd be pretty cool, but I'd prefer a newcomer Pokemon.
And lastly, Snake. I'm not too big a fan of 3rd Party characters, personally. That said, I do really like Snake, and so my desire for him to return is very situational. If there's already too many 3rd Party characters, like if Smash Switch is just a Smash 4 port, than I think adding more would just make the roster worse. If Smash Switch is its own thing, and we don't get swamped with 3rd Parties, I'd definitely support his return.