More Uncle Dante, lots, lots more. Lady, with an expanded version of Gunslinger.
From a gameplay perspective:
Expand upon what 4 did, and don't leave the game half complete. Fleshing out Nero, both as a character (either from him confronting his past, or expanding upon his relationship) and from a moveset perspective are both welcome. If neither of those points are going to be addressed however, then I'd just as soon see him be set aside.
Offer more Devil Arms, and guns than 4 had, in addition to fine-tuning weapon switching. Either allow the player to set their weapons up via the menu, or statues like in the 3, and allow for switching between 3 Devil Arms, and 3 Guns.
Expand the movesets of Devil Trigger, either each form being tied to the weapon in the first slot, or allow it to be customized slightly, like in 2. In addition, diversify Devil Arms, so that their commands are not all the same.
In terms of story, there are three directions that I'd take. Go small. Have Nero confront his past if he's present, tying in Dante, while Dante plays the trickster/mentor, while Trish and Lady try to make sure he follows the right path in a more traditional manner (Nero gets the serious parts, Dante gets the lighter stuff), as well as expanding upon Trish and Lady and Kyrie, and how they help her understand some of the issues that comes with the job.
Go big if Nero's not present. Have Dante face a serious threat, and put the world, and the people around him in serious danger.
Go Dark, and show us why Dante's the way he is in DMC2.
I'd also like to see a return of the interesting enemy designs, as well as showing them off much like 1 did in its bestiary. --- "Showing off is the fool's idea of glory" ~ Bruce Lee
All I'd want is just to make Styles more of their own specific thing while retaining quick switching. And hell, even the ability to customize it. Like up, down, left, right would be the normal styles that 4 had but there's also an option to put in, say Dark Slayer for Up Up, Quicksilver for Down, Down, stuff like that.
Also, give taunts more priority than just a little style and DT boost, let them do stuff like making enemies afraid or angering them depending on the style meter.
Slap on DMC3's Bloody Palace and DMC1's style of enemies and I'd be sold.... That's pretty much all I want, could not care less for Story or how the characters are handled. I play DMC for the gameplay, and nothing else. --- "Oh, so using my brain is cheap and dirty? My, seems I'm quite the back alley whore then." - ...A guy whose name I don't know.