A "Made for TV" movie would have been cool. I also have wondered if classic H-50 could have flourished with a theatrical extension beyond the TV show - think Mission: Impossible here, or something of that nature. Although obviously the TV reboot has served CBS very well.
If I had to simply draw a line at an existing season and say that's it, I would have to choose Season 9. Obviously some of the season's latter episodes would need to be re-done to accommodate the end, and I know this would in some cases throw the baby out with the bathwater for certain episodes of seasons 10 through 12, I think it's a pretty decent cutoff point.