It could be amazing. You could customize Dante, equip him with things that determine his speed, power, and so on. If it were an open-world RPG in the vein of Lost Odyssey and the old FFs, the scope would make it all the more epic. Imagine seeing a huge castle like the one in DMC1 or DMC4 in the background, with (randomly spawning?) enemies in between like in Dragon's Dogma. Maybe some interesting places like in The Elder Scrolls... seems a good idea.
In any case, there's a dire lack of RPGs nowadays, and I think an RPG with combos from DMC could be really nice.