Doh! I knew I was missing something! System Shock 3 of course. You could practically have System Shock 3 already keeping Doom3 just the way it is. Just throw in an RPGish interface
Also, AvP3 with FarCry engine would be nice for Predator. Would be perfect. Though Doom 3 would be perfect for those marine/alien missions. More specifically the marine though. I'm sure a mod will come out that will add aliens into the DOom 3 story mode or something. The enviroment looks a lot like the Marine campaign in AVP2. Cheers!
"This my finest sword. If in your journey you should encounter God, God will be cut." -Hattori Hanzo ~Kill Bill