The theme is about as open as it comes mate, I don't know what kind of variation I could put on it Gish? Two classes. Theurge? Two classes. If you want to combine three classes, so long as you can take three class features (or more, one from each) and find ways to combine them for fun and profit, then by all means do so. It just makes it more complicated to write.
Has to be base classes; no combination of prestige classes please. Kind of takes what little "prestige" that prestige classes have left to them out of it.
-X