Since the topic of the Forbidden Seven came up, I wanted to say that I think that Ultimate seems like the time when Sakurai will give us the Forbidden Seven (or most of them) and Gematsu. From the Forbidden Seven, we have everyone except Dixie and Toon Zelda/Tetra and from Gematsu we currently have everyone except the Chorus Kids.
-Dixie Kong: I think has a good chance of being added into Ultimate in the base roster whether as a unique character or Diddy echo
-Toon Zelda/Tetra: I think Toon Link was chosen over Toon Zelda during development of Brawl. Toon Link was added late in development from what I understand and I doubt there would've been two Wind Waker reps in Brawl. I think Toon Zelda has the least amount of a chance of being added into Ultimate.
-pra_mai: Plusle and Minun seems to be the speculated character, but I heard on Source Gaming (I think) that pra_mai could've actually been Pichu. I think that Pichu being pra_mai makes sense as we would've gotten everyone from Melee back in Brawl (with the exception of Young Link). Pichu obviously is back in Ultimate now.
-Chorus Kids: I think they (or a Rhythm Heaven rep) have a strong chance of making the Ultimate roster. Especially since Chrom is in Ultimate now and the rest of the Gematsu leak was in Smash 4 and now Ultimate.
So, I think Dixie and Chorus have a strong chance of being in Ultimate. If Pichu is pra_mai, he is back. However, if it's not, P+M has a very low chance of being in Ultimate. Toon Zelda/Tetra has a low chance of being in Ultimate in my opinion as well.