there aren't many biomes yet so I have a few suggestions:
mesa
hills
swamp
mountain pass (it is a mountain that has a large passage way that has a few ledges on the sides of the passage to put ranged units)
beach (a half of the map is sand and the other half is shallow water)
island (in the middle of the map is a island and around it is shallow water)
island 2 (a bunch of small islands and some make formations.example:a snake formation of islands.the rest of the map is shallow water.)
cave (very dark and sometimes ((I think its stalagtites it might be stalagmites)) fall from the top of the cave killing any units under it and there are a few small ponds of shallow water)
seasonal biomes:
Halloween
valentines day
easter
thanksgiving
optional biomes:
france (because why not)
rome
greese
norseRussia (because why not)
Germany (because why not)
that's all I can think of.also for the modern spin-off there should be kaytushas.