Marvel Studios has been building an expansive universe populated by iconic comic book characters for almost a decade now, and most of the storytelling and been tight, thorough and fan-pleasing.
The comic book giant's movie arm has adapted and handled most of these heroes and villains with the utmost care. Robert Downey Jr is Tony Stark and the character's arc has been largely true to the comics, if you can overlook most of Iron Man 2.
The same goes for the other main Avengers, but this isn't always true of the MCU's supporting cast. Some characters have fallen into the cracks between sequels or found themselves caught up in plot threads that lead to oblivion.
This is a problem that dates back to the first phase of the MCU as several characters from the pre-Avengers era are still missing in action, but it's an issue that persists. In the latest movie, Thor: Ragnarok, several key characters from the previous films were AWOL and, in some cases, their omissions were felt.
It's possible that Marvel will reveal the fates of some of these absentees in future films, but others could will be lost to the ages.