Make Dante and Vergil main characters on the next game.
It would be great if each of them has different levels instead of two characters sharing same levels kinda like RE 6
This, so very much this. Vergil needs a comeback (and I'm pretty sure Capcom knows this too, considering they brought him back for the DMC4:SE), he and Dante make for a fantastic dynamic. Nero has run his course, he had his time and should be retired with grace, limited to minor appearances like Lady and Trish (though there is great potential for Vegeta+Trunks-esque shenigans with Vergil).
Two playable characters with their own missions should be the absolute maximum, bonus characters for NG+ aside, otherwise the experience will be too fractured. 60% Dante/ 40% Vergil sounds like a good split, with both facing each other as bosses several times. But without level retreading unless it's how it was done in DMC1 and DMC3, no more doing EXACTLY the same level backwards, it only hurts the game with both the critical as well as the fanbase reaction.
Otherwise keep the gameplay exactly as it was in DMC4 with the environments and story heavily inspired by DMC3, go from there and refine it (So no QTEs, dumb and funky puzzles, Rebellion (fav character action weapon for life), Uncle Dante, Pizza and the best hack n'slay gameplay in the business), but if Itsuno is developing this then he has my full trust. The game HAS to be developed by Itsuno.
DmC2. Much better series than the original
You could fill an ocean with so much salt.