What DMC needs is more variety in its art design. Meaning more dynamic lighting, more variety in its environment design. That means colorful levels, more contrasting colors in all parts of the levels, as well as more visual darkness. Not just grit and grime and flat colors.
I'd love more varied and creative environmental art - the Saviour's eyeball hallway and the opera house aside, 4 didn't really have that many environments that were as distinct as 3's endless void and infernal river; or the mirrored courtyard and distorted cathedral from 1. And while I don't necessarily want to see the DMC series completely abandon its current aesthetic, it could definitely take a lot of inspiration from its contemporaries to further realise it.
DMC5 needs more human skulls embedded into things.
Human skulls are overdone. The obvious thing the series needs is more dinosaur skeletons - I can't believe they only stopped at one...