You know what'd be cool...
End of DMC5 Bonus ending: Dante and Nero (Or whoever, let's just say Nero for now.) is in a pizza parlor chatting and eating pizza, Nero is exhasted. They finish up and decide to take their leave.
Dante: You need to get some rest kid, you look like a zombie.
Nero: Pfft! Yeah right, any nutcase would know that zombies don't exist.
Nero is so tired he hardly notices his surroundings and bumps into a man wearing all black with black shades.
Man in black: Watch were you're going boy. (In a British accent)
Nero is too tired to even bother with this, "human."...Or is he human?
Nero: *snort* Whatever man.
Sadly only people who have played RE will have got that joke, but something like that would be cool anyways. XD
But if there is guess characters I think it should be something like that. A quick cameo in a bonus ending or something, not someone through the whole game. That'll just mess with both series. Then you'll be expecting Dante to be in FF or SF or whatever.