I vastly wanted the Miis. I didn't expect the specific iteration, or for them to be a Smash-specific character, but I'm happy with it.
If customs go, only Palutena and Mii Fighters should have them. I think customs are good for the game, though. I just wish they were more competitive-friendly.
As for returning vets; there's not many cuts that seems likely. The 3 clones could be cut, but just as easily could stay. Corrin probably won't stay relevant long enough. Roy however might due to being pushed, so him being on the base roster feels somewhat likely(regardless of Lucina coming back or not). Mewtwo is the only Pokemon I could see being cut yet again from the base roster, but the reason doesn't feel likely either(which is how hard he was to make. That's why getting him in Smash 4 took so long. Albeit, he has provingly been low priority, so who knows).
Wolf would be nice to have back, but I he was a semi-clone made last minute, and if there's no relevant game, I can understand him staying gone(unless DLC matches the timing with a game release. If Zero wasn't significantly delayed, he probably would've been DLC in 4). He's much easier to remake/make, than, say, Krystal(as noted by Sakurai), so I don't expect her either. I think Star Fox might stay the same. Pokemon, like Fire Emblem, has a good chance to get a new character, which is also why cuts sometimes happen, to avoid having a huge roster size. That said, beyond Mewtwo, the only Pokemon that could potentially see a cut is Greninja. The rest are heavily safe otherwise for various reasons(mostly extreme vet status, which matters a lot).
Ice Climbers feel obvious unless technical issues happen again. 3rd parties are up in the air bar obviously Sonic and Bayo since Sega are buddy-buddy with Nintendo. I feel all have a great chance to come back from 4. Snake is a "who knows".