After having seen Avengers: Infinity War and considering all the possibilities and watching lots of YouTube commentaries on the subject of Avengers 4, I have quick theory concerning Adam Warlock.
Kevin Feige and the Russo brothers have all stated in their various public announcements that Adam Warlock will not be in Avengers 4, because it would require having to explain his back-story to legitimize his involvement. Again, the character of Adam Warlock traditionally wields the Soul Stone in his forehead, which is currently, literally in the hands of Thanos. Hence, the character will be incomplete if he shows up in the movie without the stone.
My theory then is that at the end of Avengers 4, the Gauntlet will be taken from Thanos and the stones will once again be separated from it. I theorize that the Soul Stone will then end up in the hands of the Aakons (Yellow Skinned Aliens from Guardians of the Galaxy Vol 2). This will allow the queen to then complete her incubation project and birth Adam Warlock for the Guardians of the Galaxy Vol 3 movie set for some where in the year 2020.
This sequence of events sets everything up nicely and requires little explanation about where Adam Warlock came from. Secondly, if some of the Guardians don’t make it out alive from the Avengers 4, then the Guardians Vol. 3 movie has an opportunity to cycle the roster of heroes to rope in Adam Warlock since he is an original member of that team.
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