Will was the original test subject at Hawkins Lab
In season 1, Will somehow managed to survive and evade capture in the Upside Down for several days, and was even able to communicate with his mother after he realised he could manipulate electricity in his home dimension.
If this theory holds, the REAL reason Will was able to survive for so long was because he created it himself, which would make sense, considering the Upside Down seems to destroy everything and everyone else who enters it.
The only way this could be plausible is if Will was an early test subject at Hawkins Lab, which would make the Upside Down a side effect from the lab workers testing his psychic powers.
Will is not under attack by the Mind Flayer
This theory also suggests that rather than the Mind Flayer trying to attack Will, he is actually trying to protect him, as Will would be a 'god-like' figure in this world.
Does Will's connection to Dungeons & Dragons prove this theory?
As all fans of Stranger Things know, Will feels a definite affinity with Dungeons & Dragons, with some Redditors believing that this proves Will's ability to imagine alternate realities with expert precision.
What does this mean for season 4?
If this theory is correct, Will could be the key to finally cutting the link between the Upside Down and the world of Hawkins Indiana.