Will Sora still be a possibility for whenever the next Smash reveal(s) happen? I'm praying my hardest for that to be the case. Imagine all the callbacks they could make in his trailer, what with both Cloud and Sephiroth being playable now.
Speaking of which, if I had to make a logical guess, I think that the next character reveal(s) will be during E3 2021 in early June. Pyra and Mythra weren't too long ago (it certainly feels that way, but it really isn't), and I think that they'd probably want to save the next reveal(s) for E3 in order to make a "comeback", so to speak, and bank on E3 hype since E3 last year was cancelled. Thanks for that, COVID...
It's a tough call on Sora being a possibility still. On one hand, he is an incredibly popular character and he'd fit really well into Smash. The main problems are that so far there hasn't really been any precedence of a third party company having more than one representative in a pass / DLC cycle. Sephiroth for Square does seem to stack the chances against Sora, but there's 1) nothing saying this is actually a rule, just an observation based on prior entries, and 2) the possibility that Sora could be attributed to Disney instead.
I personally would love Sora, I'm just not sure I feel his chances are good at this point. They're definitely not 0, but I dunno, I think it would be more surprising to me now than say, a year ago.
As for the next character reveal, I definitely think E3 at the latest will be the next one. Steve was in October, Sephiroth was in December, Pyra/Mythra in February, so going by that 2/3 month cycle, that's April or May. Because that's so close to E3 in early June, it makes sense. While I don't think it necessarily probable, I do think they could do a 2019 E3 and reveal the remaining 2 characters at E3 in Hero / Banjo fashion, with a Summer and Fall release date like before. I'm not expecting this necessarily, but wouldn't be surprised. I do think the 2nd pass will be finished by Fall 2021 either way. Whether they'll announce further DLC or not by then, who knows.