Might not be the newest thread about ideas for DMC5 (though I'm not sure), but here goes:
STORY
Go on with Nero, who learns where he came from. I suppose it's still likely he's Vergil's son; maybe Vergil tried to create a son rather than have sex with a human woman. His goal would've been to create a combination of his and Sparda's DNA. Or he's just a normal son of Vergil... it wouldn't fit the symbolism in DMC4, but whatever.
Nero will be the one outside of Dante's shop at the end of DMC2, revving his sword, possibly hoping to fight Dante or fighting off demons. He'll go into hell to find Dante and possibly fight side by side with him for a short while. Maybe Dante is actually imprisoned, and Nero's origins tie into freeing him somehow.
Nero (or Dante) should defeat Mundus's son or daughter - this would explain why in DMC4 it says ''Mundus Vivendi''. Not literally perhaps, but at least it makes some sense.
Make Dante a little more like his DMC1 incarnation, or find some way to explain why his personality changed to what we see in DMC2. Maybe Lady or Trish died or is missing? I dunno. I'd like it if Lady or Trish would at least kick some ass in this installment.
GAMEPLAY
More customizability, and the return of costumes. Why not give Nero a Sparda costume, or a DMC3 Vergil costume, or both? Dante can have his DMC2 and DMC1 outfits aside from his normal one, which is a cross between DMC4 and DMC2 clothing if possible.
Nero should be a tinkerer - I mean, he did have his sword customized. Maybe make some way to return to Dante's shop to customize? Nero could change the Red Queen by improving its damage but making it slower, giving it blue flames in Exceed (like Vergil's color). Can also make it faster but weaker, giving it a red flame (like Dante's color). Nero should also get a throwable spear, as I think that's the kind of weapon we've been missing. Dual katanas could be nice too. Nero should be able to use the Yamato normally after getting his own real Devil Trigger -- the Vergil DT he's got in DMC4 can serve as a Majin form.
GRAPHICS
It should be gothic like DMC3, but some parts should be more upbeat just to avoid it getting stale. Think DMC4-like, or maybe more manic like in DmC. At least for hell a manic/demented style would work. A cross between DMC4 and DMC3 could be interesting. The engine running the game should be more like the one used by DMC4, which was a great PC engine.
SOUNDTRACK
The soundtrack should go back to the quality of DMC1, though I had no issues with some combat music like that in DMC3 or DMC4. Again, a combination could work. DMC1 OST track 5.... aaaaaw yisss.
All this without westernization, of course, as this isn't a reboot or spin-off.