It might be that a lot of people would state that a solo movie for Thanos wouldn’t be needed at this point since it’s already been seen that he’s thrashed the MCU and then been given his comeuppance in return when Iron Man snapped him and his entire army out of existence. But there’s a lot more to Thanos that a lot of people, even some fans, might not know about. Born on Titan as an Eternal, he was almost killed by his own mother since despite being an Eternal he did have the Deviant gene in him that made his skin purple and gave him a seriously messed-up appearance. This also gave him the ability to absorb and use cosmic energy though, which obviously has been omitted in the MCU since he’s seen as a great fighter, an okay tactician, and someone that’s simply powerful beyond measure since he can smack around the Hulk and Thor as though they’re children. Whether people want to believe it or not, Thanos is even tougher in the comics, and he’s a lot more demented since he’s been through quite a few things in his life that helped to shape the psychosis he’s been working with for so long. Almost being killed by his mother was just one of the many things he’s had to deal with in his life, as Thanos apparently started out being far more timid and didn’t enjoy the idea of bloodshed in the least.
That obviously changed though when he started dissecting animals and then his fellow Titanians to figure out what made them tick. He even killed his own mother in an attempt to find out what had made him emerge as the being he was. So at some point, he did end up ditching his dislike for carnage, as it’s been evident that while he’s not a bloodthirsty brute, Thanos is still not someone that shies away from death, carnage, and destruction. In fact, as many people that have read the comics know he’s had a serious thing for Lady Death, even if she doesn’t manage to return his affections. The entire start of the Infinity Gauntlet story started off with Thanos killing off half the galaxy just to please Death, and when that didn’t work he went on to do even more to make her happy, though since this didn’t work he would eventually create his own mate, though he still pined for the affections of Death. At one point in the comics Death even denied Thanos the ability to die, which became problematic since throughout his long life Thanos had been enhancing himself through various methods, so it would happen that the mad titan would become even more powerful and have the advantage of not being able to be killed since Death had spurned him.
There’s a lot of material to work with when it comes to a solo movie with Thanos since he’s had something to do with a good part of the Marvel universe and has interacted with so many heroes and villains that it wouldn’t take too much effort to tell his story, and perhaps even tell of how he assaulted his homeworld and turned it into the blasted wasteland that was seen in Infinity War. Considering how long he’s been around and how much he’s done in his life, it would make sense to bring Thanos back for a solo movie. It does feel that it wouldn’t be possible to bring Adam Warlock into the fray however since he hasn’t even been introduced yet. But there are plenty of other stories to tell, such as how he molded Gamorra into the lethal assassin she would eventually become, and how he mistreated Nebula so horribly. There are just too many stories to tell to not allow Thanos back since he’s been a destructive force in the Marvel universe for quite some time. Even pitting him against some of the more powerful villains and heroes in the galaxy might be kind of interesting since really, in the comics he smacked Captain America once and that was it. Obviously that wouldn’t have been great for the MCU since it would have been a short fight that only a few of the Avengers would have been able to survive.