Have you played Epic 2?
No, I have not, and I'm on the fence about it. I love DOOM with all my heart and soul, but I'm not actually the most proficient player, and the comparisons people draw between Epic 2 and Plutonia and/or Hell Revealed put me off a bit.
Icarus is right up there with the best. Definetely! :)
I'm working on a mapset with a lot of Icarus inspired levels, although you'll have to wait until next year to play it. :/
That's excellent! ...About the levels, I mean. Not having to wait. I always wanted to see an Icarus sequel, but actually done in the style of the original. Again, not knocking Daedalus, but while it retained certain elements (exploring a ship, eventually going planetside), it had notably more puzzle emphasis, and just didn't seem to have as much variety in level design. This is probably largely due to having to retain a cohesive flow from area to area, and because there were no simulation levels. A real "Icarus 2" would be very neat.
Edit: Oh wait I forget, there's two mapsets for Ultimate DooM that have a space travel theme. Base Ganymede E1 + E2. Both are pretty good episodes.
Trying out E1 now. I kinda like it. Different than your typical DOOM 1 mapset. The enemy count is more in line with DOOM II, and the health/ammo balance is really good--you're not exactly hurting for supplies all the time, but you're never exactly stocked to the brim either. It's also fun to finally play an E1 set that isn't a bunch of mappers going "lolz lets do knee-deep in teh dead again!" (No offense, guys, but I think DOOM 1 can still be good for things other than remakes.)
It's a bit harder than your usual DOOM 1 set, though. I'm up to E1M5 now, and they're just throwing everything at you. And the whole set has possibly the most annoying Imp snipers I've ever encountered, always seemingly placed just far/high enough away to circumvent DOOM's auto-aim.