I like Dal33t's idea, with blue pips. It would make it look the card look so very sweet. Maybe an ice blue, or something (Flaming anime blue, Maybe?)
As for designing, who is going to be designing the cards? Professional arteests or could we help. I could help with that, if needed.
The backs: Hmm... I'm really thinking a vortex of blue flames on black would look hella sweet. Cards themselves should definitely be black.
Jokers: I like the idea of a demonic joker, like him (or her, haven't seen a female joker yet) be dancing in blue flames, or something.
Face cards: I suggest making whatever face cards there are be the same color as the pips, hearts, clubs, or spades that the face is on. (I.e. if the pips are flaming anime blue, make the king flaming anime blue as well.)
Ace o' Spades: Again, going with a demonic firey look, make the Ace a big blue flame, maybe with little tongues of flames coming off.
Gaff'd cards: I dunno if these are up for suggestions, but I think it''d be cool if one of the face cards was a gory old zombie/demon/skeleton. It'd be cool to have the king o' spades on top, give it a rub, and it changes to the corpse of spades.
If you need any help designing, I could help with maybe the ace or the back. Oh, and if you need a beta tester for your cards, I can do that too!
...Seriously.