This is quite an interesting question. We know there is one upcoming fan game (I forget what it’s called) where Lego explicitly gave the creators permission to use the Bionicle IP. Since they’re probably not making any money off of it, Lego isn’t gonna send them a cease-and-desist letter. But as for losing the rights entirely, well…
Lego was the company that created Bionicle, right? And they filed a copyright for it, didn’t they? And the fact that Lego hasn’t made anything Bionicle-related for some time now isn’t because they don’t want to. It’s because the sales eventually declined, and continuing the line wouldn’t be a very smart thing to do. But here’ the thing: they still copyrighted it. It’s their thing. Until the copyright expires, they can do whatever they want with it. Theoretically, they COULD lobby to have it extended like Disney did with Mickey Mouse, but Bionicle is nowhere near as popular as Mickey Mouse, so I don’t see that happening.
That said, if Bionicle did become public domain, then that would open the door for even more fan works. And, as others have mentioned, the fan works could actually make money without Lego suing them. In fact, Hollywood could make an actual, big-budget Bionicle movie if they wanted to! (Though they may go to Lego to make a line of sets to tie in with the movie.)