1) Green Jungle, Pros, could be fun and if done right will look good, Cons, maybe to much work with several armies the same.
2) Grey/Brown rock like the colonial world in Aliens, Pros, will go well with the Xeno force I plan to make, simple, easy and fast, Cons, may be a bit bland.
3) red rock, like Mars, Pros, easy and fast, will give a good scifi feel, Cons, if I get the colour wrong it could look terrible.
4) Urban rubble, Pros, will fit well with a planned Terminator force, could look stunning if done right, Cons, a lot of work over several armies.
Any help or advise will be welcome, cheers.