I like all of the ideas you have here. The Empire nation would be cool to have since I already have the Avernum nation you released. And the other nation ideas sound very strange indeed. (Non preaching priests? Waaaa?) So just clone yourself and work on all of them at once.
But I guess if I had to choose, I would go with the Jomon idea. Mainly because the Jomon nation as is is sorta bland (besides their summons, I guess) Plus, you do real good work on replacement nations. Iron Faith Ulm Reborn is fantastic.