Plastcraft just put out for pre-order their new sci fi terrain which looks to have been designed specifically for Star Wars Legion. Take a look: Plastcraft Sci Fi

I think the terrain's aesthetic fits perfectly in the Star Wars universe, although the price seems to be a bit high compared to what their other terrain ranges go for.

As 3rd party kit go. I love these guys - some of the 4Ground is quite good also (I prefer the prepainted terrain because I'm lazy).

That's really nice-looking, but holy cow you're not kidding about it being expensive.