I'm already hyped for a possible Space Marine 2 and hope Relic gets the chance to make it. Some varied environments and maybe a little but of Dawn of War 2 thrown in could pay serious dividends.
Customization and wargear? Very light squad management? Maybe as simple as just bring down AI controlled reinforcements, turrets, using buffs, etc. It could work, and mix up the gameplay.
In my dreams though, this is a perfect candidate for Left 4 Dead's dynamic spawning. How awesome would it be on replays if you didn't know when and where you were going to run into another Ork horde?