I agree that Alan Ruckman's passing is going to make capturing the vibe and atmosphere of the movie difficult. It might be best not to depend too much on the old cast in general, but the concept is sound. There's several different ways anything new can build on the foundation of the movie. I think the best way to go would be to focus on the fans that helped in the end and see how they're doing this many years down the line, and then get them in contact with the Thermians somehow.
But anyways.... Bottom line is that Galaxy Quest as an ip certainly deserves a future.
While doing research, I read that in 2018, one version of the sequel would have replaced Alan Rickman's character, Sir Alexander Dane, with the character's son, Alexander Dane Jr. If you were gonna do that...I hate to say it, but Zachary Quinto comes to mind as an able heir to the character.
*takes deep breath*
By Grabthar's Hammer, by the Sons Of Warvan, YOU SHALL BE AVENGED!"