Someone like Loki? Or Thanos himself. I'm thinking of someone horrible because when Marvel say it's a risk, you know it's gonna be something stupid and gimmicky. Like Tony Stark being his dad or something.
Clearly it's not Tony Stark, which would make him full Terran.
Loki makes no sense, or reason to tie him in to GOTG. That story would be a whole new subplot requiring explanation and time they won't spend screen time on.
Whether the GOTG super fans like it or not, I think it's going to be a pivotal character in the Infinity Gauntlet story, which would make you think Adam Warlock.
While other characters like Starfox, Starhawk Mar-vell are cool to the GOTG franchise, the MCU is not going to bring in all these people.
They already have Captain Marvel, they aren't going to then bring in Mar-Vell, two characters named "Marvel"?
If you bring in Starfox, then you have to further explain the Celestials, the Eternals, the back story and how he is Thanos' brother. Which takes up time they don't have.
I think too many in here are looking at this from strictly a GOTG point of view. This is part of something bigger and is all leading to the Avengers: Infinity War. All these newer characters with baggage that are not essential to the IW story line don't need to be brought in. Again, Adam Warlock fits best and brings in another infinity gem.
What annoys me is James Gunn's repeated statements or idea that GOTG is its "own thing." He said the Guardians and the Avengers would never cross over, etc. However, Gunn obviously doesn't run Marvel or the MCU.
Bottom line, I'm not saying it HAS to be Adam Warlock, but I believe you have to look at what best fits the MCU story line and what character ties into IW best.