Speculations inspired by everything I've read here:
The Mind Flayer wants to interfere with our world and to do that it needs a spy/illusionist/antenna there.
Can't be Henry because he doesn't look human anymore, there's not enough left of him. Will is the one: he shares similar backstory and traumas, he was plugged into the UD in s1 and survived
For his plan, MF needed Will to be fully plugged into the hivemind too (ep4)
Other things were needed, not sure which ones: the spores people have been breathing in Hawkins? The particles interacting with the radio tower? the tower being plugged into the abyss? Will being near the whatsitsname huge energy ball thingy in the UD? Will's emotional link to Mike being erased? Memories messed up?
IF they're back in our world, my guess is the "illusion" or whatever MF is doing is still limited to Hawkins. That's where the spores could make sense: inhaling them made people more receptive to the MF deception. Also I like the continuity with what was already there: Hawkins in quarantine, now cut out from the world on another level, the UD wall reflecting some invisible wall in the real world… (and I loved WandaVision, so)
El is still alive somewhere outside, they need to reach her to escape. Mike is the one who could notice something's wrong but he needs to find his feelings/memories first. He needs to tell a different story.
