I put arctic cos I love esamir on planetside 2 BUT there needs to be variety, one of the strengths of planetside 2 is that its got 3 very distinct biomes to fight in with very different types of area requiring different tactics.
I've always enjoyed a fight in the desert, the ice and occasional an urban environment. So to have this kind of chance in the Warhammer 40,000 would be a welcome chance.
I think a Jungle planet would be great for ambushes so that I can embrace the path of Kauyon, They won't know what hit them...
My greatest dream is an urban setting, especially a hive city. I would love to see something like the siege of Vervunhive in Gaunt's Ghosts recreated in game. Failing that, a death world jungle is fine too.
Perhaps it could be made as "dynamic weather"? Devs said they are testing possibility of dynamic weather. Can you imagine, guarding your fortress, no targets on horizont, and then fog starts to come down, and when it lifts 15 min later, you see few hundred attackers with vehicle support few steps away from your "safe" fortress... Or even use it to move some small squad trough enemy lines to disrupt their supply lines in background. Hopefully devs will manage to make dynamic weather possible!
Urban areas without a doubt. Otherwise I think mountainous areas littered with ravines, tunnels and rocky outcrops would be ideal to provide a really interesting battleground that places emphasis on close-range encounters over vehicle/long-range combat. Anything to break up the lines of sight and make battles feel more like a hunt.
It'd really love to see some Death World Jungle style world (thin Catachan), but I fear especially jungle terrain would be enormously taxing when it comes to system resources.