Create a large map of Oz, including all major landmarks from the books and movies (e.g., Jack Pumpkinhead's house, the living apple trees), custom factions and races, magical effects, technologies, sovereigns (Wicked Witch of the West, Glinda, and so on), monsters (e.g., winged monkeys, which you can control if you wear the magic golden cap)...

I thought I'd heard about a Conquest of Oz strategy game a few years ago, but maybe that was just a dream I'd had.  I think this would be awesome.

The big hurdle is research.  I haven't read all the books, and I haven't memorized every detail of the ones I have read.  It would take a lot of work to make something truly accurate.  But, then, accuracy is something that could be postpones until version 2.0.