Third, looking at DisneySea, I'm not so sure a loose common theme is always a bad idea. That park managed so perfectly to complement the anything-goes Tokyo Disneyland park with its "exploration" thing.
I think that's the key; as long as the theme is elementary enough (so not "California" but "discovery" or "adventure" or similar,) it has the potential to be a great park. And maybe that works better in the end than having two Magic Kingdoms next to one another.
But well, they did hit the jackpot with the DisneySea concept. They're not likely to find another one of those. "Global exploration from an Old-World perspective" is of course the perfect counterpart to the exploration-from-a-New-World-perspective concept of Disneyland.
Hmmm, did I just stumble upon Disneyland's real theme?