The ideas are very nice and thematic, even if I personally wouldn't like to play with so many different kinds of "water terrain"... So for the land we should create Jungle, Savannah, Permafrost, all the terrain variety around the Mediterranean, and much much more... the game seems me already so difficult! But for you experts' mods and maps it would surely add tactical variety.
I can't agree just with one thing: River low in sites? o.O Rivers have been magical places for tons of legends and religions. Just think about King Arthur, or the godly aura around the Nyle and the Mesopotamian rivers... any kind of magic shall be present in the proximities of a river imho.