"What if" the MCU retconned old movies into their universe?

So, we've heard for years people whining that Spider-Man is at Sony and not Marvel Studios.
"Spider-Man should be in the Avengers!"
"Spider-Man should be in the MCU!"
"Marvel should get back the rights to Spidey!"
No matter how right these outcries may be, the simple fact is - he's not, and he won't be anytime soon. Get over it. But still people ask, what if his films could be revealed to actually exist in the MCU?

I'm done beating that dead horse (for now), but recently I started thinking: what if we took a film or franchise...not an ongoing franchise but one that's over and done...and found a way to retcon it into the MCU?

So that is my challenge for you. Which previous Marvel film or franchise do you think would be the easiest to retcon and say is actually part of the MCU, even though the MCU didn't exist yet, without causing too many continuity errors?

For me, the answer is the Blade film series. (I haven't seen the entirety of the TV series so I can't account for that). Because of the supernatural element to the Blade story, the public is largely unaware of Blade and the vampires he hunts. That could explain why it seems like nobody knows about him or ever mentions him...because he operates in secret. And he is so focused on eliminating vampires and again, supernatural threats, he would be too busy to care much about other heroes popping up, he's got his own job to do.

Obviously this isn't something I expect to happen. I just enjoy thinking of the possibilities.

What other films do you think could - hypothetically - be retconned into the MCU?

Discuss!