Avengers: Infinity War - 10 Possibilities For The Secret New Character
6. Adam Warlock

Let's deal with the elephant in the room first: James Gunn says Adam Warlock won't be appearing before Guardians Of The Galaxy Vol 3, where he's likely to be a key part of the narrative. So expecting him to appear in Infinity War might be a little redundant.
But then this wouldn't exactly be the first time a film-maker had lied, and you have to wonder about the timeline issue between Guardians Vol 2 and Vol 3. As the former's narrative ended in 2014, you have to wonder what exactly Ayesha and Adam would be doing for the remaining 6 or 7 years before the next sequel is rumoured to be released.
That's an awfully long time to sit on a grudge and keep the gestation of her weapon secret from the Council demanding to know why she's wasting so many resources chasing the Guardians. It just doesn't fit.
In that respect, it would be better to see Warlock introduced in Infinity War - even just as a cameo that establishes his problematic morality - especially as he's such a key part of the comic book arc.