Slitherine

Looks alien enough. Since. the depiction is a 10,000 ft. view, and something is green you can't really tell if its alien. Also, I think if you're playing for long hours you don't want too bright of colors, like pink and orange, and don't make the screen too busy so its hard to tell if you have for ex. a road. Its hard enough telling if you have units in cities somethimes. I would rate it as good.

A mix of the two should be great....

Terrestrian zones, with some weird alien zones full of secrets... or antique cities...

How easy will it be to mod the game? If players can create new terrain graphics easily I wouldn't think this matters too much.