The Marvel Cinematic Universe is a vast, interconnected web of over fifteen movies, so it's somewhat inevitable that certain characters and plotlines will get lost in the mix.
With so much going on, some things just get left by the wayside, and as audiences respond to each new film, other things are changed or added. As a result, the universe's stories are constantly shifting, and plenty of threads are left behind along the way.
Just look at how Pepper Potts disappeared between Iron Man 3 and Spider-Man: Homecoming - despite being a huge part of the Iron Man trilogy and The Avengers - or how Thor has been moved away from his Earth-based escapades.
It's clear that the MCU isn't entirely set in stone, and something that works well in one movie could be completely absent from the next.
And while the issue of Pepper's absence was addressed with a few snippets of dialogue here and there (and her eventual return), there are certain plotlines the universe has abandoned - whether to pick them up again later on, or simply as an attempt to bury them completely.
10. Samuel Sterns' Fate
The MCU has abandoned plenty of characters - Darcy Lewis, Jane Foster, Lady Sif - but Samuel Sterns' story is one of the more overlooked, exciting prospects the cinematic universe didn't fully exploit.
His first (and last) appearance was in 2008's The Incredible Hulk. Near the end of the film he's attacked by Emil Blonsky and suffers a cut to the head, with some of Bruce Banner's blood seeping into the wound. The wound starts to bubble and mutate, and as Sterns smiles at the camera, we leave him. Forever.
This moment was clearly planting the seeds of Stern's transformation into The Leader, a villain from the comics. The Leader has green skin and an oversized brain, so the shot of Sterns' forehead expanding was almost certainly put there on purpose.
In The Avengers prequel comic titled 'Fury's Big Week', Black Widow actually encounters Sterns, who now has increased mental capacity - but has also gone a little crazy. Sterns is taken in by SHIELD, to be studied by the organisation's scientists.
But hold on a second... SHIELD was disavowed after Captain America: The Winter Soldier, with remnants of the organisation eventually coming together a few years later. So what happened to Sterns? How could he still be held captive after SHIELD crumbled?
There could be a super-intelligent individual with a giant head just wandering around somewhere, and that's a weird thread to leave dangling.