To be honest I found the Drive Forms always more like a flashy gimmick and KH 2 was in general too easy with too many overblown flashy stuff.
Up to now, the gameplay system of the series had something new everytime, so I guess we can assume the same for KH 3 as well.
On the other hand though, the Drive Forms give Sora something unique, just like TAV have their personal Command Styles (Dark Impulse, Ghost Drive, Wingblade) and I would actually like to see if each of the characters has some unique vibes and techniques available only to that character. That would add diversity and prevent redundancy that is already partly there with formely "unique" spells like Meteor Crash, Faith or Salvation being available for several characters.
The only technique I can name on a whim that is still unique to one character only that no one else has is Riku's Dark Aura, as Spark/ra/ga doesn't count since it's new in DDD.
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As for new costumes, SRK aren't the only ones that could use one.
Lea should preferably get something entirely new, as always the black coat gets boring.
(On that note, Naminé and Xion could also use some new garments if we're at it while Roxas should use his private clothes more than black coat).
Then there's also the issue of TAV with their 10year-suffering break. Would be neat if they change their outfit as well in the later parts of KH 3.
If they would really allow costume change controllable by the player, the old outfits could be of course still be kept available.
When I saw KHDDD I was dissapointed they replaced drives with dream eaters and flow motion D:
