Good Vibes Gaming did a discussion video on who are going to be the last two fighters for Smash.
Hmmmmm, let's see... First I'm going to discuss characters I DON'T think well be part of the Pass, in order to narrow things down.
For example, while I think a Gen 8 Pokemon is possible, I don't think it would be the one revealed at E3 unless both fighters were to be revealed in the same event. I also don't think it'd be the final fighter, so if it were the fighters were to be revealed on separate dates, then the Mon would have to be revealed in May.
I also don't think Square or Microsoft will be triple dipping in the same pass (though I do think there's a chance they'll double dip), and Sakurai stated there won't be any echoes in the Passes.
Others I don't see happening:
Ryu Hayabusa: The lead developer of Ninja Gaiden (and possibly Koei-Tecmo as a whole) stated multiple times in separate interviews that they (as in Koei) have not received an invitation yet, and FP2's contents were finalized in November at the latest, before that final interview.
Assist Trophy promotions: While I'm keeping an open mind for them being possible in the same game, I don't think that would be very likely, especially after Min Min's addition (which is something I've stated more than a few times).
Another Fire Emblem character: We all know that kind of character reveal would yield a lot of outrage.
Now, onto candidates I do see happening as E3 reveals: I think the chances of it being a 3rd-party are high. I also see a Breath of the Wild 2 rep as a possibility, since that game is also likely to show a lot of its details at E3.
I also see a second Sonic rep, such as Eggman, as likely, since not only would another Sonic character be E3 material, but someone else also mentioned that such a character, with examples like Eggman, Infinite, and the Avatar, would make for a great tie-in to the loose-ends theory due to repping Sonic Forces, especially if the character were to be Eggman and his stage would be that ruined city from that game, complete with giant Badniks in the background and/or even stage hazards.