@Grey Acumen, I agree a lot with your points, but I’ve seen you say “Vortigons” a couple of times now…you know it’s spelled “vortigaunts”, right?
As for the Half-Life game series, I think it really should end with defeating the Combine. We should also know most everything we wanted to know, with maybe a few questions still around to debate about. It should be satisfying yet still somewhat mysterious.
Then, with the series tied up, Valve can either quit making games in the Half-Life universe, or they can keep making games in that universe but with different settings and protagonists, like Portal. Either way, Gordon and his conflicts should stay confined to Half-Life, at most getting a reference or a tiny cameo. The rich background of the universe is great but new games will feel stale if too little changes.