I think it should end with Gordon returning to Black Mesa. WAIT!. Not some kiddy time-travel thing where you can change the future, but consider the theory of a universal record of past, present, and future. The game ends with you on a tram in Black Mesa, but its a bubble of time that repeats itself (this bubble of time is something the G-man can't change, thus getting out of his influence). The screen fades and says something like this.
SUBJECT: Gordon Freeman
STATUS: Escaped sphere of influence
The scientific theory (forgot what it is called) states that time doesn't exist, and is an un-erasable record of all events.
SUBJECT: Gordon Freeman
STATUS: Escaped sphere of influence
The scientific theory (forgot what it is called) states that time doesn't exist, and is an un-erasable record of all events.