Just because the contract is completed doesn't mean there can't be more games. The next half life game could begin with G Mans plan Phase 2, or even him without a plan. Basically, theres not many potential endings that could happen without a possible sequal. Gordon dies? Time Travel trick, hes back in an alternate timeline. World Destroyed? Freedom Fighters escape to Xen and try to rebuild a semblance of a life with the vorts, opposing any combine that have traveled there. Combine utterly Annialated? Bring back Race X, or another sentient race, or some government conspiricy that was forced underground. Basically, if a franchise needs to be continued, you can discover a way, particularly when theres a lot of technology or magic.