O I said after, i want to said before, but now, I don't know what is the best option, because in Half life Opossing force Black Mesa explodes by a Atomic Bomb (if i'm not wrong), but maybe some part of black mesa doesn't take any damage, and the scientists of that part tried to create a portal to left Black Mesa, the player is the first volunteer, the machine explodes, and the player starts to travel to a lot of places, he see some vortigaunts that they're not slave, and he start to help them for rescue the planets that Nhilianth had took...
Yeah, the idea of use goldsource and source its great.