A lot went down here while I had my sleep, huh...
I think there is a low chance of Dixie being in Diddy's Final Smash but there is a chance. I don't see her being an AT though.
We first need to know whether Dixie Kong has a major non-playable role or not in Ultimate. That is the biggest contingency to whether Dixie Kong even has a shot at becoming DLC for Ultimate. We have only 40 days left until the game's release, and the final pre-release Nintendo Direct will likely happen at least a week prior to the game's release. It could happen as early as next week. We might very well be around only ten days until we know for certain the remainder to this game's roster.
Regardless, for Dixie Kong, a concern is whether she could possible end up as being part of Diddy Kong's Final Smash or a new Assist Trophy. We shall find out in the weeks ahead, for better or worse.
Sakurai considered Chrom for playable status, but then thought he was too similar to Marth and Ike, and wanted something "fresher" to represent Fire Emblem.
I do not rule out undesirable scenarios for Smash Bros., because a lot of the time they do come true.
My problem with this assumption is that if this were the case for Dixie's final role, she would have already been shown be part of Diddy's Final Smash
months ago - most likely shown in E3's Smash Ultimate Direct Showcase. That's where all the Final Smashes that feature new cameos from characters' teammates (
), had managed to be shown fully prior.
I'd find it rather cruel and strangely unprofessional of Sakurai if all he decided to do was to pointlessly hold off showing Diddy's Final Smash due having Dixie in it up to this point of time - since Dixie being in it just deconfirms her outright anyway, so there'd be nothing to hide about.
Otherwise that'd be leaving Dixie-fans hanging for months upon end with Smash-speculation, only to just drop the ball on them while Fiora-fans and like get to process their wanted character's deconfirmation more safely. Like I said, that'd be unprofessional, and strange to target one particula fanbase of a demanded character with (Toad at least seemed to be shown to probably not be playable way back in E3 again, since he was shown to be more involved and active in Peach's moveset now.)
Also, if Diddy's Final Smash was kept in the wraps because Dixie was suddenly being corporate to it, it'd imply she'd been pretty long while in development. since up to E3, the game's been halfway done with and had a full demo build done for it.
The best conclusion I'd think why she'd be dropped her to a Final Smash, instead of just keeping on working on her as DLC, would be that she somehow couldn't work, or Sakurai deemed her "not interesting" or "potent" enough for a compelling moveset, much like what happened to Chrom.
But again, I think comparing Dixie to Chrom here is little bit comparing apples to oranges. Chrom certainly was very similar to his predecessors, hence why he was dropped in favor of more unique newcomer in Robin, and got luckily reworked as Roy's Echo instead in Ultimate.
Dixie however, I think we've seen more than enough proof that Sakurai doesn't see her as a mere "Girl Diddy" - and even then, to deem her beyond merit or time to be put on just drop her to a Final Smash, would go against all of that.
It could probably happen, but I bet it'd come off to many as frankly illogical choice in the very end, given how Sakurai currently has a full-blown options to ensure return of veterans that he had back in Sm4sh, or even introduce more 3rd Parties, thanks to the great age of DLC and free add-on content. So it would have made ton of sense to people to just let her get finished after game's release, and bring her in as DLC.
So the end line would be that if Dixie was in Diddy's Final Smash, then Sakurai and his team just didn't her workable - even if it'd be hard to believe with the insane amount of characters in Ultimate, and some of them being even semi-clones themselves. A lot would probably call bias ("Sakurai liked Isabelle more") due lack of understandable reasoning on her exclusion - at least it wasn't rocket science to see why Chrom didn't make it over Robin, and Echoes being around would make Sakurai's reasoning on Dixie's exlusions worse.
Maybe it'd be the Ridley-situation where Sakurai felt trying to bring Dixie in without making her feel like she's not herself at all was difficult - but I'd be hard-pressed to wonder what it is about her most iconic trait and attributes that'd make her so hard to work with.
So yeah, I think if Dixie was made into Diddy's FS, I think it wouldn't feel "logical" or "good" choice like Chrom was done with - it'd just be a straight-up cop-out move from Sakurai's end, especially after trying to make playable Dixie a reality for a 2nd time.
(I just realized that I'm now wondering what in earth could be Diddy's new FS be about, if it'd have Dixie incorporated to it now. Some have suggested him using Animal Buddies like Rambi while Dixie trails along - or maybe it'd be Kong Pow from Tropical Freeze? )