I think when Dante finally snaps, he should be in the plaza, with a whole bunch of angels surrounding him, mocking him. Then, after a few seconds, he goes ballistic. I think he should look like the Dreadnought armor, but with gold sleeves and leg armor, a red, coat-like armor on his chest, and large shoulder pads. Oh, and wings. Can't forget the wings. It would be called RDT. Rage Devil Trigger. Oh, speaking of triggers, I think Dante's trigger should change depending on his weapon/style. Like this...
Trickster: Dante's DT is very sleek looking, and wears an aerodynamic helmet, with large spikes coming out of his back that make him look like he's moving extremely fast. It can use very fast punches and kicks, and he talks very fast.
Swordmaster: A large, armored form with a sword grafted onto the arm. Slightly slower than Trickster form, Dante becomes a powerhouse, smashing all that oppose him. However, due to his reliance on his sword, he cannot use guns. His voice is deep and commanding.
Gunslinger: Dante's personality reaches an all time high with this one. Although he can still use swords, they're weakened, and lose their elemental properties. However, Dante becomes faster, and his guns become strong enough to be used as normal weapons. For the player who likes ranged attacks. Dante's voice is similar to his DMC 3 DT voice.
Royal Guard: Imagine Swordmaster form, but even bulkier. This is Royal Guard form. Although he's extremely slow, his defense is unrivaled, and he can use both swords and guns. However, they're slightly less potent, due to his slowed speed. His voice is extremely dark in this form, and you cannot see his face. It's hidden behind a helmet.
Devil Magic: A form that uses the elements to it's advantage. One arm constantly changes from fire to ice, and the other drips water, yet arcs with electricity. His wings are formed of light and shadow. His voice is slow and deliberate. His swords are formed from the element you choose. He is also the only form that can use magic.
And, if they're coming back...
Quicksilver: A black and white DT, and you can't see his face. It's covered by a helmet again. He's very fast, and talks so quickly that you can only understand him if you slow down time. Oh wait, you can!
Doppelganger: Normal DT. Only changes once the Doppelganger appears. Big, spiky, menacing. Never talks. Need I say more?
Good 'nuff?