Replaying MoM for the 1st time in a while, & yeah I stand by my belief that this isn't the "complete compendium" Nomura made it out to be. Now that Spoilers are off, I might as well go over the obvious things that can be added.
1. More KH3 songs
Number 1 with a bullet is the lack of KH3 related content. Only a few songs made it into the game with most boss themes missing, the battle themes for Corona/Toybox, the world themes for Monstropolis/San Fransokyo, & both for Arendelle (plus the labyrinth) being the big omissions.
& while it can be pointed out that it would be impossible for such worlds to exist in the Switch version, we have seen locations & character designs specific to KH3 redesigned for Switch. I don't think it's completely unreasonable to recreate the Disney worlds in the same manner as Keyblade Graveyard & Final World. Lord knows the films those worlds are based on have had games on Wii, PS3, & other weaker already & still turned out fine.
2. Port Royale/The Caribbean
Special note has to go to the PotC world, of which both its KH2 & KH3 iterations were left out of the original game. One wonders what happened in dealings with Disney that Square couldn't use this world in MoM despite appearing in KH3 a year before.
3. Tron/Jack Skellington/Prince Phillip/Quorra/Quasimodo/Neku
Speaking of odd absences, some worlds lack their guest party member for whatever reason. (Although I suspect they just straight up forgot Phillip was even in BBS.) Would really like Square & Disney could sort out these hangups & get the MIA Disney elements back.
Now I know what you're thinking, "Neku/Quasimodo/Quorra weren't ever party members" Yep, & it sucked back then too. They totally should be upgraded to guest status at least in this game.
4. Daybreak Town/Wreck it Ralph
So far there's been no Union X/Dark Road tracks, not even from Back Cover I think. With WiR having about 3-4 sub-worlds, each with their own tracks (depending if Grand Central Station is included), there would be at least 10 or so tracks that could added. This would again require the creation of new environments, but it would really be nice to see the WiR sub-worlds in 3D.
5. More Bosses
It's downright baffling how few boss stages are in this game. Either they take a lot more work to implement or they were an afterthought/low priority to the main field stages. But still even then only 4 boss stages just isn't enough (even 0.2 technically had more). If I were to hazard a guess which ones to add:
-Young Xehanort for DDD (L'Impeto Oscuro)
-YX/Ansem/Xemnas for KH3, reusing assets from the earlier stages (Forze Finale)
-Dark Chirithy (Dread of Night)
-Marluxia for Chain of Memories (Lord of the Castle)
Would've also liked more Disney bosses aside from Maleficent, but it may be asking too much. Maybe King Candy to continue the UX representation.
6. Kairi Team
& of course I couldn't very well end this hypothetical DLC/sequel post without the biggest zolephant in the room: Kairi's utter mess of an arc. While it wouldn't make things right with her (that ship has sailed), it would at least make it better if the new training was covered in the DLC & done with instead of dragging it out in the next major game.
Best guess for teammates would be Lea & Pluto. Lea didn't exactly do a whole hell of a lot better with Xemnas shattering his keyblade & all & probably needs more training himself. Lea would differ from Axel by having his keyblade & his new clothes. Pluto is the closest thing Kari has to a Disney companion in KH2 & it'd be nice to see that played up again.