I would do what DMC4 toyed with in Nero's progression and go full SOTN with it.
Both Dante and Nero would exist simultaneously and independently of one another in a upgrade-gated Metroidvania-ass environment with their own goals to pursue that can be freely swapped between ala GTA5 outside of specific events. To ease travel as environment grows, portals are present to the player.
On Dante's front, Gunslinger crazy combos return (screw Devil Arms) with all other styles starting out where DMC4's maxed-out Dante style movelist ended and developing from there with progression-unlocked upgrades like Wall Runs, Doppleganger, Quicksilver, and weapons that allow Dante to go deeper into the environment. Ultimately, Dante would grow past his DMC3 styles strength and levels of ********ery and beyond that into a new realm of absurdity for his combat options.
Nero follows the D-Pad weapon switch ala DmC. Exceed system is expanded to each weapon to offer unique functionality to add significantly more tools to Nero's game. New gun's dragged in to compliment Beast Rose. Nero's non-weapon/DB focused upgrades fixate on upgrading Nero's available interactions with environmental objects that are present in much greater density than DMC4. Nero's an angry brawler with style, I want to see that played up.
And then throw in a SOTN Richter-style Vergil campaign where he's tearing through everything like a bat out of Hell with DMC3 Vergil's weapons expanded into three real full-fledged DAs, DmC-style Angel/Demon Pull Swords available to him and two DT modes; Normal and Nelo Angelo which is triggered similarly to DTEs by charging orbs for Nelo Angelo time.
/bonus: Played character gives the non-played character **** every time they pass each other through the environment and even feature taunts calling each other out for their horse crap.
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.
I'd much prefer that any elemental customization be moved to Devil Arms, honestly.
They should just forget DMC2 ever happened and just make it non canon.
They pretty much already have. The game has been moved so far into the future and it has zero risk of contradicting anything in subsequent games to start with because, well, nothing happens in it anyway except establishing that Dante has a new sword. The only reference DMC2 ever gets are like a single name in a DMC3 cutscene and whenever they start diving into it for random assets and mechanics to re-contextualize in DMC3 and 4.