It would make sense that Warlock would appear. In Guardians of the Galaxy, we saw his cacoon, and in Guardians of the Galaxy Volume 2 we are getting Ayesha AKA Her. So there will definitly be a connection to him.
Warlock could be trapped in the Soul Stone.
According to this James Gunn clearly stated that no Infinity Stone would appear in GotG 2.
Well like I said before, maybe he is in the Soul Stone.
Or maybe he is somewhere out in the universe and he'll be the one to get the Guardians of the Galaxy to fight Thanos.
He doesn't need the Soul Stone does he?
Are we even sure either Adam Warlock or Magus will appear? I know he was added as a character on the Infinity War page, but I followed the link to the article that mentions him and it's been taken down. So does it even count as a source anymore? And this part is just my opinion but does it even make sense for Warlock and/or Magus to appear right now? The movies have been building up Thanos' involvement for years now, I'd be disappointed if he was suddenly upstaged and replaced as the main villain by the Magus appearing out of nowhere.
I was thinking about how Marvel would introduce Warlock into the MCU, and Doctor Strange gave me an idea. In Doctor Strange, Mordo has the Staff of the Living Tribunal. Thus, they acknowledge the Living Tribunal's existence. In the comics, the original, or at least the first known Living Tribunal died. The position was then open. The position was filled by Adam Warlock of Earth-14209. And since there is only one Living Tribunal in the entire multiverse, if the MCU is somewhat concurrent with the comics, Adam Warlock does exist in the MCU as the Living Tribunal.