-Allow for more environmental input when it comes to combat. Like in the reveal trailer when Dante threw the car at the demons.
-Make Devil Trigger more like the original with it not blasting enemies in the air with a time freeze but a power and speed boost that offers up its own pause combos and attacks with it also not draining so fast.
-Allow the player to strip weapons from enemies to perform crushing finishers. For example, fighting a demon with a scyth and countering at the right time will allow an opening to hit a button stripping the enemy of his weapon which you can use as either a projectile or for one massive finishing blow.
-Incorporate the parkour like moments from DMC 2 that had Dante running up walls and stuff.
-Add more of the gunslinger style moves like the one where he rides demons like a surf board only also let the player jump off that demon to let them slide in to others like a bowling ball to pins.
-And a wider variety of guns and get a little wacky with them. Make up guns if you have to. Possibly even let players dual wield two different types of guns and different shoulder buttons control the specific triggers.
-A weapons crafting system. Upgrade and customize weapons and guns to create Dante's own personal arsenal which can even go out in to more of a loadout thing before missions. Kinda like how you picked styles before each mission however, no styles. Keep the hybrid combat systems. The crafting and loadouts will just put variations on the one combat system. So you will still have all your toys but, mixing it up with your own personal touch to it all.
-A combo creating and sharing tutorial mode. Games are moving in a more social direction and while I say no multiplayer simply putting in the inevitable combo videos and tutorials in to the main game just for that community seems like a very convenient and natural decisions.
-New weapons, preferably the addition of an actual whip or what TwoX put in his fan fic, a kusarigama or things like duel wielding sai blades and a spear.
-Grappling moves. Just a quick grapple button to possible toss some enemies around or even depending on the direction you pull it in, can be like Dante hoping over the enemy to get behind and break his neck.
-Bullet time. Let me explain, you can have a meter that allows you for a little bit to slow down time and focus your shots to then let go in real time to a spectacle of a glorious barrage of bullets Dante style. You can get more immersed in the gunplay as yo pick how you shoot them. Anyone play Stranglehold? Something like those special bullet time maneuvers.
-A mission creator mode. Test all your skills in a mission creator mode that lets you as the player plant demons and other challenges in missions for others to complete. It would extend DMC's replayability even more and also, let them customize their own personal Bloody Palace towers. You want to create the most brutal and unforgiving boss fight galore Bloody Palace? Do it. See who survives.
I know I talked more about features in general than just combat but, I believe the ideas are pretty sweet.