Wouldn't it be hilarious if Marvel decided to kick the legs out from under Universal's attempt to create a classic monster "cinematic universe" by just knocking out a classic monster team-up movie of their own set in the MCU?
It'd give them an excuse to re-introduce Blade and I'm sure they have some Werewolf type characters that can tap. No shortage of mad scientist experiments either of course (they could even use Hulk!) and I'm sure it wouldn't be too hard to find stand-ins for the Mummy and the CftBL either.
They don't even have to use stand-ins. Marvel already has Dracula, Lord of Vampires and N'Kantu, the Living Mummy as Marvel characters.