Marvel sent out a "Hero Teaser" image which included the text "A Long Awaited Return..."
We see the image also reads: "I knew this was my chance to be a hero."
The image looks a lot like Quasar's star chest symbol, and we also see the "A" in "Standoff Assault On Pleasant Hill" is the symbol of the Avengers.
Quasar was a member of the Avengers.
However, there is another chance it's for Genis-Vell, but his star symbol is eight-pointed.
"Standoff" also has to do with the Thunderbolts, of which Genis-Vell/Photon was a member. It's possible Marvel may have simply mixed up the symbols (lol) or tried to act coy.
I'm also not sure if the teaser text fits with Quasar, unless it means his return to the Avengers.
Anyway, it looks like we'll find out who it is next week with the release of Avengers Standoff: Assault on Pleasant Hill Omega #1. Here's the solicit description. Maybe Cap called his old buddy Wendell for some help?
S.H.I.E.L.D.’s clandestine secret prison Pleasant Hill lies in ruins. The villainous Baron Zemo and the super villain inmates of Pleasant Hill stand upon the verge of victory. Should S.H.I.E.L.D.’s reality-warping technology KOBIK fall into their hands, the Marvel Universe will bend to their will.
All that stands between them and total annihilation are a small band of Avengers. But who is that leading them? None other than the restored Steve Rogers! With the original Captain America at the helm, they just may stand a chance. And seeds sown here will set the stage for major things to come in the Marvel Universe.
The return of Steve Rogers, the formation of the all-new Thunderbolts and more! The can’t-miss ending to Avengers: Standoff!