What if... the anime trigun was a sequel to blue gender?
Trigun takes place on an entirely different planet, doesn't have any of the same themes, doesn't have similar technology, doesn't have non-humanoid monsters, and none of the humanoid characters have the same or similar abilities between the series. Unless these are literally the only two science fiction anime you've ever watched, it makes no sense to try to think of one as a sequel to the other. You might as well say "Expelled from Paradise" is the sequel to "Sasami: Magical Girls Club," for all the more they have in common. The ending of Blue Gender alone means you would have to rule out all anime where humans are spacefaring or about to embark in space travel before considering it as a prequel to Trigun.
Ugh...no. Blue Gender was awful. Trigun was awesome. Please don't put that in my brain.