A quick look at Google and you’ll find that as recently as 2014, Galaxy Quest is still being talked about by the cast. There’s speculation of a script somewhere according to Tim Allen. Justin Long has “never given up or surrendered” the idea.
There are numerous ways to approach a sequel. There is a Wikia site dedicated to the movie (no surprise), with information on the “TV episodes” and the Thermian history. If the cast is all for it, the fans are for it, the reboots of Star Trek and the upcoming Star Wars are successful, why not?