The fifth element is a perfect self contained story. I’m not sure how you could up the stakes for a sequel. You could tell other stories in that world maybe, but I don’t think a sequel featuring the original characters would be good.
Valerian and the City of a Thousand Planets was something of a spiritual sequel by the same filmmaker, but it turned out to be an unentertaining bag of cinematic butt.
I found it to be rather entertaining, I liked the action, I liked the design and I liked some of the charactets. Unfortunately both the main characters and the plot seemed rather underdeveloped to me, so it kind of did turn out to be a bag of cinematic butt.
Yes, they did come off as siblings! It would be so much better if they were just friends. The sibling chemistry was there. I also found it strange that in the end the leading guy was supposed to finally stop playing it by the book, but he never really did during the whole film. That was a whole missing arch. And the bad guys were sleeping most of the time.