For DMC6, I'd want an optional DmC styled control scheme. More moves for every character, including Dante. Every character being playable on every level. More customization on the sound track... If I want 3 classic DMC songs to play at random, let me do that... As well as allowing me to replace boss music - looking at you, Vergil.
Bring back regeneration for Super Costumes, don't degrade the style rank outside of getting hit. Allow me to retry more quickly, instead of having to wait for animations or for menus to merely appear. I'd even be happy with a shortcut command, L2 + R2 + start or something. Just give me something to get to the start of the fight more quickly.
Bring back items, and rework the revive system to compensate. Vitality / DT stars, holy waters, untouchables, etc. Repopulate them every playthrough - some of us actually like exploring these levels more than once. Replace blue orb fragments with gold orb fragments once collected. Items and gold fragments becoming red orbs once collected, but reverting back to items / gold when all other stages are beaten. Essentially becoming like DMC1 - you beat the game, and all items come back... Except, you can play the levels in any order.
I'd also want DMC6 to experiment with a new direction, and place more emphasis on atmosphere. I think remaking / reimagining DMC1 would be a great way to go back and experiment with that formula. Maybe at first just have Dante be the only playable character... I wouldn't want him to have his styles. I'd want them to take their time with him, and figure out what other weapons he may have found on Mallet Island. Really spend time to simplify his mechanical complexity without reducing the number of moves available. Make the DMC1 remake the introduction to the series that DmC should have been, and maybe alternate between the two formulas if they strike gold with it.
They could also have Trish and Nelo Angelo as DLC characters, but I'd seriously want them to take their time with them as well. I was disappointed with Trish and Lady in DMC4:SE. I'm disappointed with Nero as well... I want a lot of moves, and I want easy access to those moves.
Of course, some players want it to be mechanically difficult to do for the pure sake of it being difficult, and that's okay too... but for the next game I want them to start from the beginning, and rethink it all. Have DMC7 come back to the mechanical complexity that those players crave for. I genuinely think they can achieve it without creating a low skill ceiling like DmC.