My turn.
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Before you say anything, Sonic is the most likely to return for me. I simply decided to put Bayo on the left because I believe she's a different case than other third-parties due to being a ballot winner and as such,
can't really be predicted in the same way as the others. This means that she isn't necessarily more likely than the other third-parties, just on a different category that is
independant of them. Now that I've explained myself...
Sonic is practically guaranteed. Sega is really friendly with Nintendo, Sonic has appeared in two Smash games so far, making him a recurring veteran, etc. Really not much to say aside from that, honestly.
Yeah, Pac-Man looks a bit weird, but let me explain. If Namco works on the next Smash game, he's as much of a guarantee as Sonic and likelier than Mega Man. If not, he's as likely as Mega Man but less likely than Sonic. This is because all of them are huge, recognizable icons of gaming, but can have different circumstances depending on whether Namco has a role to play in Smash 5 or not.
For the Capcom reps, I could see both make it in, but since Ryu seems to be trapped in Sony-exclusivity with SFV and Mega Man is getting a multi-platform mainline game with MM11, I'd say the blue bomber is a bit more likely, but not by much.
Cloud is the least likely out of the non-Bayo third-parties from 4 simply because the other companies actually have their IPs being put in Switch games, so licensing is much easier. He could still come back, but since mainline FF games never show up on Nintendo consoles, it's a little harder for him. Not
that much harder since Square Enix does put games on the 3DS and Switch, but still a little bit harder, especially since we barely got any FF content for Smash 4 in the first place.
At first, I put Snake for the lolz, even adding way too many ">" between him and Cloud to troll the Snake supporters, but personally, I could see Konami be fine with putting Snake back in a future game. I just don't know how likely he can be, especially since the one reason he got in Brawl was because Kojima asked, so I put him as the least likely to play it safe.
As for Bayo, she's a weird case. She only got in because of the ballot, so it could be a one-time oppurtunity, but at the same time, Nintendo is funding Bayo 3, so they clearly believe in the franchise enough to put their money into it. And with Sega being on such good terms with the big N, she could be as likely as Sonic, the most likely third-party, but could also have a chance to not show up as well if the ballot was a one-time deal.