Keep the broad original categories:

Mobile Monday
Ta Ta Tuesday
Wet Wedneday or could be Free for All Wednesday
Tushy Thursday
Sexy Legs Friday

And let the individuals be creative.  Anyone can take anything and make it boring or they can be creative about it.  

I may be wrong, but someone having to plan ahead is going to get old...

Keep it simple and who ever wants to be the theme day star will be the one who puts the effort and thought into it.

Just my thoughts and opinion.....I like to participate sometimes and when I can but I certainly don't think my life depends on it.  It's supposed to be fun.