A wiser and less angsty Nero as the main character. Don't take that as an insult against the character -- I think he had good demonstrable growth in 5 compared to 4 (and generally I liked him character-wise compared to 4, where I kinda loathed his personality), but I want him back in 6 as the central character again, but feeling older and a bit more mellow, bit more of a leader rather than cocky young'n living in Dante's shadow.
Trish and Lady as playable characters in the main narrative.
Playable Dante and Vergil... But probably not in the main narrative. I wouldn't mind them making an appearance in the narrative, but the Sons of Sparda don't need spotlighting anymore, IMO. Reserve their appearance mostly as honor to the series legacy, but not really as the focal point any longer.
V's playstyle as a Style Switch for Vergil.
A series of events spanning multiple locales that are feeding into a larger plot. There's been a good amount of variety in DMC's environments but they tend to exist within one cohesive region -- I kinda want the characters to sort of globetrot around the fictional world of DMC to completely distinct locales as the story progresses, with the events of each locale feeding into a larger premise. If there is one thing that kinda felt weak in DMC5, it was that everything with the Qliphoth felt a bit samesy, and a lot of the game took place there. To Capcom's credit, it was NOT as egregious as DMC4 where the game kinda folds back onto itself -- the levels were indeed all distinctive with their own pathing and secrets and whatnot, but the visual aesthetic was a bit too samesy. Very minor complaint, but one that I think could be addressed just by a change of the scope of the premise. That's one last thing I want borrowed from DmC -- come up with any excuse to vary up the visuals of the environments vastly. I think a globetrotting adventure could keep the aesthetics feeling a bit more gothic and grounded while having the variety on display from DmC's sort of abstract, otherworldly setup. I wouldn't even mind if some of those locales were revisits to older environments from previous games.