The 5 DLC characters were chosen by Nintendo and confirmed by Sakurai. This leads me to believe that the DLC characters will only be big names mostly from upcoming games, and not something like Bandana Dee or Geno, even though Bandana Dee has been my most wanted newcomer for the past two Smash games. Shantae has been deconfirmed by her creator campaigning for her on Twitter after they decided on all of the characters. Phoenix Wright is possible. Doom Guy is a longshot.
My official prediction is:
Rex
Sora
FE: Three Houses rep
Zelda rep (probably Impa)
Another 3rd party not from Square (probably Tails, Banjo, or Travis Touchdown)
No Walugi?!?! Wtf insig i thought you were a man of culture.
Also, since Diablo 3 got ported to the switch, wouldnt mind seeing a Necromancer or DemonHunter as a playable character or at least support
Keep in mind that a regular Piranha Plant was chosen to be the first DLC fighter. It makes sense for Bandana Dee to get in if a Piranha Plant could; as for Geno, it's a big maybe. I have a feeling Shantae's creator might be campaigning to keep it a surprise/make her appearance uncertain, but that's probably unlikely. As for your predictions:
Rex - On one hand, it seems like there's no way for him to get in. On the other hand, Chrom is in the game but still has a Mii Fighter costume representing him.
Sora - Possible
FE: Three Houses rep - Also possible, judging by the fact we got Corrin as one of the last DLC fighters.
Zelda rep/Impa - Once again, possible.
Tails - We do need another Sonic rep, and Tails seems like the go to character.
Banjo - Maybe if Microsoft will let Nintendo use him/if the stars align properly.
Travis Touchdown - Also a possibility, and would be really hilarious to see.
As much as I would like to see Waluigi playable, he's been deconfirmed as an Assist Trophy. Any Diablo characters seem like a longshot.