In the comic book arc Thanos Wins, you see Thanos defeating the Living Tribunal, Infinity and he says he killed all life.
That would include abstracts, Franklin Richards and whatnot, all without Infinity Gems or cosmic cubes. How did he (as the comic says) ‘grow’ more powerful?
The aforementioned arc is the only mention of this alternate universe that I am aware of. It is never completely explicit how Thanos has won (its also not clear if he ever regained access to the all infinity stones), but a number of hints are given.
He possesses a number of powerful artifacts, and presumably used others along the way:
He outlived various mortal foes and/or waited until they were old an weak
He was constantly increasing his power through experimentation on his unique deviant biology. It is implied, though Frank Castle/Cosmic Ghost Rider's own story that Thanos has spent a long long time slowly killing off beings and getting more powerful.
He exploited various other powers. In one panel you see him sacrifice Black Bolt in front of a host of Celestials(while standing on the corpse of the Living Tribunal), and the next panel shows a massive explosion obliterating the Celestials. Presumably he had some trick to survive this.
In the end though, he sees a powered up Silver Surfer with Mjolnir as a serious threat, and is eventually beaten by his younger self, suggesting that while he has increased his raw power over time, many of his previous victories probably relied on more that just raw power.
