I hope it's the Sentry, after he shot off into the sky with Exitar's corpse and with him having knowledge of the White Hot Room he is pretty up there in cosmic terms. Not saying he is Abstract Tier himself but he has a lot of knowledge and is extremely powerful.
Plus when he flew off in Uncanny Avengers #22 he said.
"I've enjoyed being an Avengers today Janet. Have no fear -- I'll take a great journey. Moving Exitar's body far from Earth. But, even then, know this... The Celestials wrath will be mighty"
Him saying he enjoyed being an Avenger could fit with the "I knew this was my chance to be a hero" thing. If we remeber after a while the effects of the Death Seed are said to wear off. And it happened with Daken and the Grim Reaper. Who is to say that the Sentry returns from deep space free of the Death Seed's influence. Or not and he retains the Death Seed persona but he has mastered it so to speak. I mean he rebelled against the Apocalypse Twins from the start so why not.
Of course the symbol in the picture looks a lot like Quasar or the original Captain Marvel symbols on their chests. But then if it was any of them 2 (or a hero at all for that matter). Why would the said character say,
"I knew this was my chance to be a hero"
It wouldn't make sense for a hero to make that comment. It either has to be a redeemed villain, anti-hero or a character that is doubting himself and then saves a group of people.
Excuse my tinfoil hat Sentry theories by the way. I always get really into discussing it when there is the opportunity.
Hugely disappointing from Marvel but exactly what I would expect... replace an established character who has his own history with some unnecessary new and lame female version. Why not create a new female character? Why not get some female creators to come up with something of their own instead of just more copies and replacements? Are they that afraid that nobody will be interested? Just give it a go, if it fails at least they tried. Anything would be better than ruining these old characters.