It remains to be seen if The Eternals will be set after Avengers 4 or, like Captain America: The First Avenger and Captain Marvel, will be a prequel to the modern day MCU.
It’s also worth noting that one of the more important characters in the Eternals mythos is Thanos, who recently made a name for himself by wiping out half of all life in the universe.
While Marvel is understandably keeping quiet about The Eternals’ plot right now, sources told THR that along with depicting the conflict between the Eternals and the Deviants, this movie will also show the love story between Ikaris and Seris, the latter of whom “relishes moving amongst humans.”
If it’s the latter, then perhaps we not only might see a younger Thanos, but also meet his parents, Mentor and Sui-San, and his grandfather, Kronos, who led a rebellion against his war-loving brother Uranos long ago.