For the latest example of a character that isn't showing up in the MCU without HEFTY changes being made to everything about them, we have Cletus Cassidy, otherwise known as Carnage.
Now technically speaking, Carnage is going to show up in the next Venom movie, but that movie series has been pretty firmly established to be non-canonical with the rest of the MCU. As for the MCU proper, Cletus is one of those characters who can't really show up without some heavy edits being made.
For one, he's a serial killer, and a shamelessly gleeful one at that. And while this isn't too out there for Marvel, let's just say that Carnage earns his name once he gets his powers. The character is a perfect walkthrough - even moreso than Venom - of just how dangerous and violent a Symbiote is, as Carnage is often shown reducing entire rooms of people into chunky salsa.
Maybe Carnage will show up in the main MCU, but whoever they get for that movie better have some clever ideas to work around the more R-rated parts of this character.