-Better matchmaking
-Longer campaign
-Explain stuff from Gears 2 (sires, how some random scientist knew the locust HQ location from 20 or w/e years before, what the hell the locust did to tai to make him deface himself, etc)
-Balance the weapons, start by removing the boomshot completely
-Host Migration, or please please please, dedicated servers!
-Special horde mode where you play as locust against AI COG soldiers
-Fight a berzerker underground so you cant just hammer of dawn or make it fall off a train...