Superman, Adopted by Thanos

Thanos definitely had genuine feelings for Gamora, otherwise the Soul Stone wouldn't have appeared. It just goes to show that Thanos was willing to give up everything, including himself, for his own dreams.

Doesn't Marvel have Superman comics?

Forget about Mind Machine Superman; that's the fusion of the souls of 52 Supermen controlling a mind-controlled robot to become a Mind Machine Superman.

Stop pretending in a single universe. Please, take a step forward and venture into the multiverse.

The universe's resources are finite. We fight for the balance of the universe. Thanos is the only one who takes action despite knowing everything. If Thanos were the protagonist of the Avengers, would you oppose him snapping his fingers? In the face of harsh reality, all ideals are illusions. Those who oppose Thanos are simply those who haven't witnessed the cruelty of reality.

He didn't reach the level of Silver Superman, Gold Superman, Mind Overdrive, or the Codex of Life; he's just an ordinary little Superman.

All I want to say is that, unless you reach the Yellow Sun, Superman is no different from an ordinary human.

In the comics, the Ancient One died and went to play with Eternity.

Actually, you could exploit a plot hole: in the comics, Superman slept in the sun for tens of thousands of years and turned gold. You could have the protagonist go inside the sun and use the Time Stone to accelerate the flow of time there. The Time Stone would definitely work for such simple time manipulation. Also, another explanation in the comics for the gold Superman is that a five-dimensional being gave him the ability to become a five-dimensional being.

The author portrayed Thanos too well towards the protagonist; if he were to betray the protagonist later, many people would probably criticize him.