Remake all 6 games as fully playable in Unreal 4 with Kingdom Shader lol
Seriously though, we know Kairi gets at least one new line, they might redub her entirely as she doesn't have that many.
IMO, having Keyblades from BbS, KH2 and Re:coded in Re:CoM wouldn't really make sense in the context of the story, the cards are all based on Sora's memories. At the the time of Re:CoM, KH2 and Re:coded haven't happened yet so Sora couldn't possibly "remember" the Keyblades he obtained in that game, Re:coded only had one new Keyblade and the rest are already in Re:CoM, and Ventus' heart is still asleep so his memories of his Keybades are out of Sora's reach. The Days Keyblades at least made more sense than the KH2 ones being in the PS2 ersion of Re:CoM as Sora's memories are conected to Roxas', although Roxas wouldn't have had access to most of those Keyblades at the time of Re:CoM. As long as we're on the subject of Keyblades, I hope this time they replace Data-Sora's Kingdom Key with Zero-Two in the cutscenes after he gets a real Keyblade, but if they didn't do it in the PS3 version, then they probably won't do it here either. I'm still disappointed they didn't add Oathkeeper and Oblivion to DDDHD.
As for what I want, I dont see any reason why they couldn't add a Theatre Mode to KH, that's something they should have done in the PS3 release. They added it to the PS2 version of KH2FM when neither the original Japanese nor international releases had it. Also, make it so Deep Dive plays after Another Side, Another Story, rather than outright replacing it.
I don't really pay attention to things like glitches and bugs as they don't really bother me, but fixing them is always a good thing, just look at Re:coded lol
It could they could take adantage of the PS4's online connectivity. The only drawback I see is that it would mean they'd buff Armor of the Master and No Heart, as in the PSP version of BbSFM they where created with 3 players in mind but where nerfed in the PS3 version to make them easier to solo due to the lack of multiplayer. Adding multiplayer back in would likely cause them to be rebuffed making them nigh-impossible to solo.