It would've still retained the repetitiveness. The story would still be about Xion rather than Roxas and insight on other Org, the missions would've still blended together, etc.
Personally I don't think anything could've helped days. Even if it had commands it still would've been a repetitive collection of missions with enemies that hold an annoyingly high amount of hp.
Magic heavy characters like Zexion or Donald really badly needed it. Once they run out of spells, there basically isn't anything they can do anymore in a mission.
However the very structure of the game's story needed to be rethought. I felt they should've gone with a Crisis Core set-up myself with a chapter for every month of Roxas' life & missions kept optional.
The panel system used in Days by itself isn't the main problem.
It is actually a pretty good idea for a battle system at the base.
It had however some flaws like Level Up-panels using too much room and (as Matgsy pointed out) was not very useful towards magic heavy characters because of that.
The main problem with Days repetiveness gameplay-wise is that a) there are very few different mission goals, b) not only do you always go to the same worlds, you also nearly always go to the same areas of that world and c) there are no dynamics at all.
Story-wise the game suffers heavily from Xion being a spotlight-stealing squad by herself and nearly every conversation even if she isn't present being about her. More interesting plotlines that may also be somtimes more important for the overall series are left hanging or even delegated to be "secret report"-material only to fill every possible bit of space with Xion's overblown woobie-story.
I'm in no way a Xion-hater, as I actually like her as a character, but her story during Days was so obnoxiously pushed to the forefront and blown into the player's face that out of the tormented ones, I actually feel the least for her because her tragic story was played up so excessively in pictures and sound that it became annoying.
The proverb "less is more" would really have worked wonders here.
I believe the panel system of Days to be a prototype of the Command Deck, they function very similarly and since Days came first, it's not too much of a stretch.
Perhaps it's because I love puzzles and quickly adjusted to the panel system, but I never ran out of magic >.> I'd always pick one specific type of magic for a mission and put a ton of it in there, between the x4 magic panels and the elixirs, it was hard to actually run out.
A little bit better, but nothing game-fixing. The game is bad. It's not a good game. The command deck would've fixed the horrible menu system, but it wouldn't have fixed the bad world design, the boring gameplay, the funky enemy AI, the (HERE COMES A BIG BAD OPINION, DON'T GET TRIGGERED!) boring storyline and characters, or the bad boss-fights. The game violently leaches off it's brand name. It looks like Kingdom Hearts, but it just doesn't feel polished like one. KH2 may be the least favorite of some, but it's got buckets of polish. If you want a good portable KH, go for Re:Coded. That game is pimp as heck.
The only thing that could've helped Days was if Nomura had stuck with it being a game centered around The Organization and Roxas' time within it, that's what we were promised anyway, but instead we got a game about Xion and... No, no, that was it.
I'm not saying that it would've made the game instantly good, but it would've been better. The way the missions were structured was pretty poor, but it was doomed to fail even conceptually. It's meant to be a series of tasks you would expect the Org members to do, this pretty much means that you'd have to go back to the Castle after every mission, making it repetitive even before we get into the content of the missions. That said, the missions themselves were pretty uninspired, more varied objectives would have been nice. More varied locations would have great too, I know the DS has some serious limitations but come on.
The panel system itself I feel was fine, it could have been polished more but it was mostly fine. Of course it doesn't compare to the command deck because nothing does. I guess for one removing the spell casting limit would have been much better, that was pretty dumb honestly.