I think I recall Civilization 2 had something similar to that, multiple maps in one scenario... But CivIV you'd have to write all that from scratch because it wasn't intended for that sort of behavior. I can't even imagine how one would go about coding something like that - so huge amounts of respect to the people working so hard on this!
Sea Cities sounds cool too, will that be giving something to the Abyssal Colonization tech then?
I'm also curious as to if the combat oddity involving Lead From Behind is still present or not (It's terribly annoying but not fatal). It's been there as far back as v26 from what I can tell, still reading through the SVN threads to find some mention of it, but nothing so far. Lots to read through x_x