Well I definitely think Ice Climbers will come back. Wolf has a strong possibility of coming back I think as well. But I'm very doubtful of Squirtle and Ivysaur and frankly would only want them if they did the whole Pokemon Trainer thing again. I'd like to see Snake return, but if he didn't come back for Wii U/3DS, I find it unlikely he'd come back for the Switch version.
Then I don't think anyone wants Pichu back, and I'm certainly not expecting him. And like I said, I'd only like to see Young Link "come back" if they completely changed him and based him on Majora's Mask and he just transformed between Deku, Goron, and Zora with Fierce Deity being its final smash, but I'm overall doubtful that transformations in general would return, and even if they did, who knows if such an idea would have even occurred to them. But otherwise, the Melee version of Young Link is too redundant, as its too similar to Toon Link, and would make 3 versions of Link in general that would all play similarly.
At this point in terms of returning missing characters, I think realistically would could expect two: Ice Climbers and Wolf. Though I'd love to see Snake and that version of Young Link.