Please note the verb: "Want." Not "Have officially declared" or "Are allowed to." Cap 2 and 3 co-screenwriter Christopher Markus desperately wants the Mechanized Organism Designed Only for Killing as the villain of Captain America 3... if he can somehow figure a way to do it that isn't absurd.
Here's Marcus speaking to SFX with his screenwriting partner Stephen McFeely:
I don't know if there's anybody that we had in the wings that we couldn't pull off. There are people that I'm always wanting to bring in. I want to put Modok into something, but you can't just drop a giant floating head in! It's not like "Oh, we have to go talk to this guy – there's something I should tell you about him first…" [laughs]. Suddenly the whole movie needs to take on that structure in order to accommodate him. I never win that fight!
"You can't just drop a giant floating head in"? Avengers: Earth's Mightiest Heroes would beg to differ:
MAKE MODOK THE VILLAIN OF CAP 3. NOW YOU HAVE NO EXCUSES.