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.