For me is urban ruins during the day with ashes covering the lights surprise attacks on mechanized groups stealth actions surprise flamers and traps or jungle during the night but above all last stand in tyranid hive structures just think of a mission where your group has to place a bomb at the center of the hive complex heavy assault with land raider and flamers followed by some time until the bomb is placed and set fighting swarms upon swarms of gaunts with some carnifexes and then a run to the stormraven to get out of that place
Makes me think of Exterminatus mode for Space Marine but with better objectives and 'nids instead og IG or Orks. I think it would be better as a death-mission. You plant the bomb but have to defend it explodes/ time runs out. Your doing this for the Faction, and by the death of your Squad cause crippling damage to the 'nids. So say you have limited/no spawns off the Land Raider untill it dies, and the 'nids get progressively stronger as the timer is running down. Your going to die, but will you sell your death at a high enough price??
I would love a fog covered jungle, even better if they have day/night rotations. loving just about everything suggested in this thread so far.
Urban Ruins have to be. It has everything from Small Streets and Backyards to open Plazas and green Parks. Allowing almost any type of Combat. But i'm fine with everything.. i only dislike Jungle a bit.
I like all the environments, time of day for me is far more important. Night fighting is may favorite, I like the heavy fog idea as well. Anything into Urban is good ie, from a jungle fight my way into a city. My least favorite would be any wide open area. Although, that could be a dream come true if they allow building siege works and engines. Long trench line to a urban area. Anyway, most likely not possible but a man can dream.