One thing I think should be kept in mind when it comes to thinking of "who comes next" with Fighter Pass 2, is that folks are looking closely at Sakurai and his thought processes.
Here's the thing though: Sakurai doesn't have nearly the same control over chosen characters at this point that he used to, or even did during base roster picks or Wii U/3DS.
Pointing out such things as "Sakurai could change his mind" aren't nearly as relevant as they used to be. It's fine and dandy that Sakurai has changed his mind, but that doesn't matter if he isn't picking characters the same way he did. Nintendo is the party with the most decision-making power at this point.
A few other things have remained true despite shifts in "power" of decision-making:
1) Every single DLC pick has been a protagonist from the series they originate from. This is simple and one could say expected. Smash is a mascot fighter/crossover, meaning it makes logical sense that mascots/cover characters would be picked for DLC. This is true among 100% of paid DLC picks from Wii U and Ultimate (Piranha Plant is not included because of reasons exclusive to it).
2) While not a disconfirmation factor, it should be noted that any 1st party choices have been veterans or the protagonist of a newer title. As we have no veterans to add, this is essentially negligible, and we ironically received protagonists from the same series with the "shill" pick.
3) Outside of veteran or "shill" picks, each pick has not only been 3rd party, but from a new series independent of other series added to DLC. This is consistent among DLC picks from Wii U/3DS as well as Ultimate.
While each character seems to be a different and unique situation, I think these bits of information are worth noting and keeping in mind while making expectations of the next Season of DLC. I've seen some folks expecting 3 upgrades, when that simply goes against the mindset of nearly 9 DLC characters across two games, across 6 years, in more than one way. I wouldn't rule anything out 100%, because that's kinda crazy. However, I don't know that there's going to be some massive shift in outlook when there are certain things that just make business sense within Smash and outside of Smash.
I'm not even really anti-upgrades. I loved Golden Sun and think Isaac would be cool. The problem for me in actively expecting Isaac, or considering him "likely" is that there's no marketing potential with Golden Sun as a Nintendo property. There's no new game coming either. He would be a pure fan pick, but, 1st party-wise, this has yet to happen.
It mainly comes down to Nintendo, not Sakurai, deciding an upgrade is worth it. The only characters in my humble opinion that even reach that, are Waluigi and Rex, yet Rex received more than most NPCs do. So basically, Waluigi is the only character I could see viably being upgraded as Nintendo's response to him not being playable, but then I question the method of additional content that Waluigi would bring with him. He's not a main protagonist. He would again be a pure fan pick, but it would again go against all previous DLC except for veterans, which are obviously a different circumstance entirely.
Eh, I guess I was really bored. But there you have it, Jonesposting at its finest!