What would i change about DmC? Oh sure, hand me a Rubik's Tetrahedron why don't you...
The Ophion grapple system could have been much more interesting. I often got the feeling that the ludicrous mobility it gives you makes it completely unfair for the enemies, who even on DMD can't keep up if you grapple between them constantly or just abuse it to perform endless, effortless aerial combos. The introduction of enemies that could counter-grapple Dante, perhaps by grabbing him if he abused the tactic or swinging him into the jaws of another enemy, could have made the mechanic a little more balanced. Then there was the way the original trailer showed Dante using Ophion to chuck cars at enemies, so using it to turn the environment against enemies seems like an idea that was teased then failed to appear.
In fact on the whole the environment's role in the combat tended to be disappointing; usually it only involved uppercutting an enemy into a hazard above you or over the invisible wall at the side of a floating platform for an easy instant kill, or if you were really really lucky, sometimes the floor could hurt you. For a game that makes a big deal out of it's Hostile Transforming Environments, it completely failed to make an interesting combat mechanic out of the idea. Imagine if the terrain sometimes lashed out, like the spectral hands you got in the old games, and you could abuse that effect by luring enemies towards it just before it attacked.
The enemy AI is generally god awful too, even by Devil May Cry standards. They telegraph for incredibly long periods of time, and by using the camera you can abuse the aggression system so that you can single out enemies so you can whomp on them with impunity. When God Hand did this it at least didn't give the player independant camera control with the right stick, because the designers realised how easily it could be abused if that were the case. So yes, much more aggressive, tenacious enemies with tactics that compliment eachother, and attack in numbers on par with Legendary Dark Knight difficulty in DMC4, that'd do nicely.
I should stop now, or I never will.