One of the most compelling things about Avengers: Infinity War was how the Russos turned Thanos into a sympathetic figure despite his villainous plan involving wiping out half of existence. That's impressive work, given how easy it would have been to make him your typical, cartoonish pantomime villain.
The key to that effect was humanising the Titan and making it obvious that his convictions came from a positive place. Thanos wasn't merely trying to kill for the sake of killing, he truly believed that he was doing the right thing for the universe. He believed that he was the only person capable of doing the difficult thing that would keep the universe from destroying himself.
Even more crucially, Thanos was right.
There have been some suggestions that he could have simply magicked up more resources to stop the universe burning out, but that's already been established as impossible by the rules of the MCU. And his prediction that life's rampant expansion in the universe would lead to a catastrophic event, as happened on Titan is entirely well-placed. Eventually, it will eat itself and something has to change for its own sake, even if that means making difficult decisions.
With all of that in mind, you have to wonder about the lay of the land in Avengers 4. If enough time has passed since the snap to rebalance Earth, so humanity starts to see the benefits of Thanos' actions (the eradication of poverty and world hunger, for instance), could we see a rise in people actually appreciating what he did?
All of the talk about Avengers 4 has mostly focused on the idea of Thanos being the villain, but what if the remaining heroes are forced to fight against something far more difficult to punch away: zealousness? Thanos has already proved himself capable of inspiring rabid followers (that's why Ebony Maw was the way he was) and humanity does have rather a history of following powerful figures blindly. Hell, the Avengers themselves are exalted as gods and they've killed loads of people accidentally.
What if Avengers 4 introduces followers for Thanos on Earth, kick-starting a second MCU civil war? Imagine how compelling it would be for some of the heroes to actually wrestle with the idea that what he did was right. To put aside the pain of losing their friends and loved ones and realising that he has saved the universe in the long-term. That would be an even better fight than the one over Bucky and superhero registration.
Interestingly, it would be particularly fitting if Tony Stark was the one to advocate for what Thanos has done (at least initially). His whole arc has been about saving the Earth from external threats, but his long-term dream would be to put a protective bubble around around the Earth from all threats to ensure survival of the species. If he was to realise that survival often requires a cull like Thanos' snap every now and then, he could end up being Thanos' biggest supporter.
Then again, he's still a selfish, emotionally-unstable wildcard and the loss of Peter Parker might be too much to see past. It wouldn't mean Thanos was wrong though.