I think Dante should have dual knives. They would be centered around fast-paced combat and 'Crazy' combos. And as for a gun, he should have flamethrower or something like that. I think it would be cool. A rapid fire crossbow would also be cool as well.
As for Nero, I think his weapons should be focused on the Exceed system but should have different elements. So since Red Queen is fire based, they could have another one (White King mayb? lol) that is centered around ice. It could freeze people and slow them down. He could in addition have an electric one, an earth one ect. I always thought that would be cool. He could throw summoned swords for a gun. Theyd be different from shooting, because these would knock enemies back, he would use them kindof like spears.
And a scythe would work for both of them imo.
@DT now THAT is a badass weapon!