The only series that really come to mind at the moment for me are these two:
1. 12 Kingdoms. This is an anime based on a novel series, and it left off on a huge cliffhanger for one of the characters, so if you only watch the anime, basically, you never know what happens to him. I don't even know for sure whether the novels resolve that or not (since I'm only on volume 3), but I'm fairly sure that not all of the content got included in the anime.
2. Basara (i.e., the political epic shojo manga Basara: https://en.wikipedia.org/wiki/Basara_(manga)
). This 27-volume epic got a 13-episode anime adaptation, which certainly didn't cover everything and wasn't very good anyways. Definitely would love to see a reboot . . . I have huge doubts about the likelihood about that, since the manga ended in 1998, but on the other hand, it got some stage play adaptations in 2012, so you never know.