For a while, you couldn't get moved for people joyfully proclaiming that the MCU has a "villain problem." And while it is fair to say that some of the franchise's bad guys did fail to make quite the same impact as the likes of Thanos and Loki, there have been far more brilliant new characters more recently than there were in the earlier days.
The likes of Killmonger, Zemo, Thanos and Hela have all been excellent, but there's a completely different problem with them. Because they're strong villains, they have to be killed off or put out of service completely for the most part. So they have to be replaced with another bad guy who then faces the exact same pressures.
But what if Marvel take a leaf out of Captain Marvel's book and recycle a bad guy? It's not like every villain we've seen in MCU movies has been killed off and some of them have a real chance of coming back in some capacity...
13. The Abomination
Last we saw of Emil Blonsky, he was helping Hulk smash up Harlem and almost gaining the upper-hand on his more famous green enemy. With his mind warped by power-hunger and the mutation created by his transfusion of Bruce Banner's irradiated blood, he was more than a match for Hulk, until he made the mistake of trying to kill Betty Ross and inspired Hulk's furious last stand.
Where Is He?
After his defeat, Blonsky was taken in by the US Air Force and held in a cryocell in Alaska under the control of General Ross. For a while, the idea of him joining the Avengers was entertained, because the MCU version of the government are idiots. When SHIELD reasoned that he was a monster, they dropped the idea and Bruce Banner was recruited instead, leaving the Abomination locked in his cell in the Vault.
Major. If he was to escape, he could be a seriously useful weapon against the Hulk. That almost happened, according to Tim Roth, and it could be that those plans to bring him back might be revisited at some point.