Benvolio from Romeo and Juliet. He was Romeo's right-hand man and best friend. He straight up disappears halfway through and is never mentioned by name again. After Romeo kills Tybalt and goes into hiding, this guy named Balthazar seems to be filling Benvolio's role - it's almost as if Shakespeare forgot what his name was - but we never find out where Romeo's real best bro went.
Was it really Terra that Beast Boy saw? Did she survive? Does she have a secret twin or clone? We shall never know.
What happened to all the actors on "The Truman Show?" A whole town full of people who had spent ~30 years living as a cast for a TV show, and the main character up and fucks off at the end of the movie... what do all those people do with their lives now?
Moody's Mom from Moody's Point. Was she ever found or was she eternally lost on that hot air balloon?
The little sister on Family Matters... She went upstairs one day and never came back down.
Phoebe’s Dad in Friends. The show makes this huge deal about him, we meet him for one episode and he’s never heard from again.
Mr. Turner in Boy Meets World. Spent an entire season with him becoming a father figure for Shawn, and the season ended with Mr. Turner getting into a serious motorcycle accident. He was never mentioned again. (Unless I’m misremembering??)
Adam from Supernatural, just alone in the pit for 8 seasons now.
Everyone else in the pit leaves but not poor old Adam.
Mentioned once in the recent meta episode but only to highlight that his story is unresolved.
Did we ever find out what was so significant about Leela? When Fry travels back in time and almost stops Nibbler from pushing him into the cryogenic tube. Nibbler makes a comment about Leela being the other one in some prophetic tone. Fry beats the evil Brains and later the Dark Ones but I feel that the comment with Leela was never addressed.
Ghost...Like he was at Castle Black with Jon DEFINITELY. But I don't remember anything about him at the Battle Of The Bastards. And hes just not in season 7 as Jon travels from Dragonstone to Eastwatch to Kings Landing.
Sokka, he was in a flashback once and his death was mentioned a few times but nothing else. Did he marry Suki? How did he die? And he was a main fucking character in ATLA.
Tom Bombadil and who / what he was, what the Blue wizards were doing during the events of Lord of the Rings and where they were ever since they went East and Morgoth's / Melkor's eventual escape as foretold in the prophecy, or if there ever was one.
Edit: added more unresolved things from the Tolkienverse
Some say that Huell is still in that safehouse to this day (Breaking Bad)
Rube (Mandy Patikin) from 'Dead Like Me'. On the cusp of a great emotional reversal of the characters personality. Then the show is cancelled. We get a movie like five years, Rube isn't even in it, along with other characters, and every other cliffhanger from the show is forgotten
As of right now, Nick Fury in the MCU.
Y'all better address my man in Avengers 4. I can't have the last time I see him be in the 90's.