DMs: What is the campaign concept or idea you've never had the opportunity to DM?

[s/LaSH]

I've got a scifi universe I'd like to game in (no, LOVE to game in - I'd like to take some complete newbies and expose them to my vision of the 51st century and see what they do), and a game system I need to playtest. I'd also like to get into Mutants And Masterminds sometime, and see how some of those full rulebooks that started flooding the net a while ago work out, if only for one or two sessions. (Deadlands, Ars Magica, Witchcraft, some freebies that seem interesting...)

As it is, little time for gaming. I spend more time developing or drawing instead.