I think the boat has passed on an Alien Nation continuation, sadly. The series ended in 1990, and the last TV movie (a ratings failure) aired in mid-1997. The cast and the crew has aged quite a bit, and it's not a property with more than limited nostalgia value.
Well, I wasn't talking about a revival with the same characters, but more like a new series set in the same reality, with maybe some guest appearances or supporting roles by some of the original cast. As I said, the original show created such a rich reality that there's potential in it for doing more than just one set of characters in one city. (The comic-book tie-ins at the time were along those lines: they weren't about the TV cast, but were about other, original characters in the same world.) Although it would be nice and nostalgic to follow up on the original cast, I'm more interested in seeing how American society would've evolved after having Newcomers around for over 20 years (or less, if you set it in the present instead of five years in the future like the original).
If you did it that way, approached it like Trek: TNG with a new series and cast in the same universe -- with a different situation because the aliens have been around longer and they and the world have had more time to adapt -- then you could have a fresh, new series while still continuing the universe of the old series.