Damn, this got 200 pages in almost 2 weeks...
Anyways, my predictions are:
Returning characters (Usually they are those characters that were cut):
Ice Climbers (The character was cut out due to hardware limitations of 3DS, but since the creator is working on a game for one console (Switch) instead of two (like SSB4), there is very high chances of them returning)
Snake (There has been Metal Gear Solid 5, and Snake is still the protagonist of that game. There is also a spinoff to continue the adventures after events of the latest entry. So there is high chances of him returning, yay)
Pichu (It was the only character to be in Super Smash Bros Melee. Let's say he was an alternate character to Pikachu and we quite liked him. Too bad he was cut out. Even if it appears as non playable character or just as cameo, I just want to see him back in SSB5 with a role instead of being only a trophy)
Bayonetta (There is the third entry in works, this time it's exclusive to Nintendo Switch, so goodbye to PS fans. She will be in Smash, I'm sure)
Any returning characters I missed, please tell me.
New characters:
Shovel Knight (That indie character failed to make an appearance in the last Nintendo Direct for SSB4. It fell behind to Cloud, Bayonetta and Corrin, so there is a chance he might make it to SSB5)
Crash Bandicoot (The remake of the first 3 games is launching on Nintendo Switch, so he might make it)
ARMS character (What are your preferences?)
Rex (Xenoblade Chronicles 2)
Helper Waadle Dee (Kirby)
Daisy (I recall Memoryman3 hoping that she will be in the game as playable character and not just some alternate costume for Peach. Since Rosalina was included in SSB4, there is a chance she might be in the game. There is a rumor that Sakurai is planning to create her as character, but I do not know if it's credible)
Characters unlikely to return:
Goku (I have seen peoples on Resetera commenting about him and cheering for him to be in game. I do not understand why do fans want him to be in the game, I have not cared about Dragon Ball)
Undertale (I don't get why would peoples ask for a Undertale character to be in the game. It will have a Switch release, yes, but aside from that, I do not think that game have had any great involvement with Nintendo)