I haven't participated in this thread in a very long time, but considering the two characters up for voting right now, I feel obligated to say a few words.
Waluigi
Chance - 50%
I'll start with a simple premise: while FP1 was chosen mid-development of the base game, FP2 was surely chosen well after the fact, which means they can and probably have re-evaluated certain characters that were relegated to Assist Trophies, Spirits, Mii Costumes, and so on. For example, if we take Sakurai at his word that Rex was in consideration for FP1 before ultimately being relegated to the Mii Costume, it's entirely possible he was back in consideration for FP2. After all, they were implementing Nintendo's entire history into base Ultimate, including every character that had ever been playable before, so certain concessions had to be made for who gets to be playable, an Assist Trophy, or whatever else, and with FP2 they now have the opportunity to revisit those decisions and take the opportunity to do even more than they did before. This doesn't guarantee we will see an AT/Spirit/etc promoted to playable, of course, but I think we'd be fools to assume they're out of the picture.
To that end, there is one character who I think is most notorious for missing the roster in Ultimate, and it ain't Geno, but Waluigi. Aside from being very popular, he's the one character I know press interviews have specifically asked about when he was announced as an AT instead of playable, which says a lot all by itself. I do think there's a certain amount of bias against him, nor do I think he necessarily adds much in terms of music/stage/spirit, but it's easy to see him be presented as the Mario Sports character and come with matching extra content. Further, Sakurai's been upfront that Nintendo chooses the characters, not him, and Nintendo surely knows just how popular Waluigi is. As uncertain as I feel about FP2 (I'll be very surprised if the community guesses are even close to the final list), I think Waluigi's chances are far too significant to be ignored.
Want - 75%
I don't particularly want to play as him, but as a such a fan-favorite character and one of the "Camelot Bros", I know I'd be happy to see him make it in.
Isaac
Chance - 25%
As before, I don't think being an AT or anything else should be treated as a deconfirmation for FP2. Now, I don't particularly want to litigate here whether a Golden Sun revival is on the way (as credible a theory as I believe that is), but even without that in the picture, I believe Isaac is one of the few characters currently an AT/Spirit/etc that had a credible chance of being on the FP2 shortlist. Few Nintendo characters enjoy the same level of fan speculation that Isaac has, and his lifetime in Ultimate speculation has been particularly memorable, including fake leaks, fan art, myriad videos, the Golden Sunday campaign, and of course the unusually strong, negative response to his AT reveal. That's the sort of attention that Nintendo can and surely did notice, and thus I think the chances of Nintendo proposing him for DLC are unusually strong. Of course, there's just as much (if not more) possibility that Nintendo has no interest in reviving Golden Sun, never mind devoting precious Smash development resources towards the long dormant franchise, but with so little information about what direction FP2's character choices will go, I think a strong first party choice like Isaac deserves the benefit of the doubt, especially over the myriad of third party choices that dominate speculation talk these days.
Want - 100%
Even if you don't remember me from the speculation days of old, I think the profile picture speaks for itself. By far my favorite potential character for Smash.
Nomination - New Zelda Character x5
With so little to go off of for FP2, I'm most interested in seeing if Zelda will ever get a new Smash character. Maybe it's just the lack of an obvious frontrunner, but it's crazy to me that we have yet to see a new Zelda character since Toon Link in Brawl.