It has come to my attention that my original reddit post may not have been the most clearly written. As such I have decided to dive deeper into the theory and the possible repercussions of it.
Much of the speculation of the post decimation universe - also known as the post snap universe -has been about the fates of those who were dusted. What if the dusting didn't kill them. Is it possible? A common critique of Thanos and his plan is that the death was not necessary to accomplish his goals with the power of the infinity gauntlet. I think its possible that rather than killing half the population of the universe the infinity gauntlet instead split the reality into two separate realities. The two parallel realities could be joined by a sort of bridge in the quantum realm. Since Scott was in the quantum realm at the time of the split he was unaffected by the snap and not bound to either reality leaving him free to travel from one to the other over this bridge. Maybe this leaves him free to travel through to the parallel dimensions created by Thanos splitting the dimensions. He would be the only way for them to communicate with the heros who were dusted. Maybe he exists in both realities and gains knowledge from both due to some cinematic form of quantum entanglement. As to why the snap potentially didn't affect him:
The quantum realm is another dimension. In marvel comics the quantum realm - the microverse - is a parallel dimension that is only reachable through magic, like a sling ring, or Pym particles. http://marvel.wikia.com/wiki/Microverse
The quantum realms time works different. A common theory is that he time traveled but maybe he just got "stuck" in time and so the snaps effect are delayed.
The quantum realm is going to appear in Captain Marvel according to several news sources. In addition it has been stated that it will play a part in the upcoming phase. Here is one. https://screenrant.com/captain-marvel-movie-quantum-realm/
Thanos is no longer supposed to be a threat. The armor scarecrow is representative of his retirement. Thanos actions threatened half the universe. This greater threat must be even more dangerous.
I have a few other ideas on how this could affect End Game. A large portion of Marvel mobile games have focused on the idea of coliding realities. It could be a story choice to allow the battles that take place in them to make some form of canonical sense. However it could be conditioning the general public- those who may not be familiar with the comic book concept of multiverses - to be ready for universes coliding. Maybe one universe must be destroyed so that the other can continue to live. This would allow the next phase in the MCU to effectively transition to a Secret War style arc.
I understand this is a long shot theory. Some have said it would cheapen the actions of Thanos. I propose that instead it does the opposite. The most common theory, that of time travel, completely undoes everything Infinity War achieved. The shock that the antagonist won. I created this theory as I find that generally time travel is lazy story telling. That being said I believe that Marvel could pull off an effective time travel story.
As an addition to this theory I would like to propose an additional bit of theory. That the infinity stones, the remains of a universe, are actually the remains of extraordinarily powerful beings. These beings could be the threat of the next phase. Beings like Galactus… More theory to come in my next theory post.