Pondering this a little, I do think several things should happen before anyone goes too crazy:
1. Someone needs to design the back, and everyone needs to agree that it works. That will form the basis of everything else, from color palette to theme.
2. Someone needs to put out a complete set of card templates, with properly positioned indexes and pips set (the pip design could be changed, perhaps, but the position and size should be consistent). Without that, you'll end up with a jumbled mess if each designer decides where to put stuff.
3. Since you will have so many people taking part, you really can't help but end up with something "transformation-like" in the end, because each person will want to do something interesting, but if you get the 2 of spades you really can't do all that much without adding some new kind of detail or art.
4. Finally, from a technical point of view, the resulting design files do need to be exactly right (in terms of size, format, resolution) or you will have USPCC giving you a big WTF when you send them mismatched files.
Could be a lot of fun! (but I don't want to manage it
)