Interesting thought regarding Avengers 4. This is just my speculation, but still, if people don't want to hear speculation about what might potentially happen, stop reading. Since it is just speculation, it isn't a spoiler. But my thought process was triggered by something I read about information that was leaked about some toys. So, here goes...
The leaked information mentioned the Avengers, Rocket and a new crew, and others fighting a "new foe" and "even greater threat." I guess there is apparently lots of speculation going around about who this bigger threat could be. But I don't think it really matters for the bigger point I am about to make. Let's just assume for a moment that there is some "bigger threat" looming--i.e., bigger than Thanos.
Now, here's the thing about Thanos: he was one of those villains that legitimately saw himself as doing something "good." He was a zealot for his cause. And he viewed his cause as a benevolent one designed, in his mind, to ultimately "save" the galaxy.
So, what if a threat arose that sought to destroy the galaxy? How would Thanos react to that? For his flaws and the twisted nature of his plan in Avengers 3, complete destruction would obviously run contrary to what he was trying to accomplish. So...what if he were to basically "team up" with the good guys? Not a traditional team up. But if there were a legitimate threat that was too powerful for him to stop, and he ultimately decided that he needed the help of the MCU heroes, that could explain the expected "resurrection" of everyone wiped out during the snap.
Again, this is obviously just speculating. But something like this would actually be a palatable way of bringing folks back, IMO. I certainly wouldn't hate the idea of something like this. To me, it is more palatable than some sort of time travel through the quantum realm, which seems both too obvious and too contrived.
Anyhow, that's my theory.