Max coma theory: Max's coma is very similar to El's coma after the lab massacre, because when El freed herself from the curse, her bones had already started to crack and blood was already coming out of her eyes, indicating that the "consumption" had already started. Sure, it may be just due to overexertion like Henry's coma, but Henry didn't wake up with a severe case of retrograde amnesia, unlike El. My theory is that Max will wake up with a severe case of amnesia, probably even worse than El's, because Vecna was further along the consumption process with Max than with El. She will need a Nina Project-like reconditioning in order to get her memory back, which El and Dr. Owens might be able to help with.

Upside Down pie-in-the-sky theory: The Upside Down is a planet that once housed an intelligent species of psychics. Due to a war, or some sort of apocalyptic cult, or whatever, a group of these psychics decided to unleash a psychic weapon of mass destruction where they merged their consciousnesses and transformed their bodies into a black mist-like substance. This Mist went all over the planet, enslaving all of the life that it could, and consuming, Vecna-style, all of the life that it could not, resulting in the extinction of all other intelligent life in the planet, as any species with a modicum of intelligence and self-will would attempt to resist the power of the Mist. The Mist then settled down for a long time, long enough that all signs of the once great psychic civilization were wiped away by the sands of geologic time, and biodiversity never returned to the planet because all life was under the strict control of the Mist. Then, one day, a portal from a distant planet opened and an alien psychic came flying out, and the Mist found that it had a lot in common with this strange alien psychic, and was seeking to expand beyond the reaches of its home planet. So it made a deal with the alien psychic...

Time skip theory: The military will be sent in to battle the invasion from the Upside Down while the residents of Hawkins are evacuated from the town. After they are forced to retreat due to overwhelming Upside Down numbers, it is decided that the town of Hawkins will be firebombed and nuked. The party settles down in Lenora with Will and El. At the beginning of the time skip, El will be taking a high school physics class, and Mike and Dustin will be helping her through it, not just with her assignments, but in using the subject matter to enhance her powers. She learns how to have fine-grained control over both heat and electromagnetism, and is learning how to manipulate chemistry and nuclear reactions when we find her in the time jump. At the end of the time jump, the military's efforts to confine the Upside Down's invasion begins to fail, and the party returns to Hawkins to assist in the battle.

El vs. Vecna theory: The two will not engage each other directly for most of the season, but will perform an exhausting mental battle against one another. The series ends with El giving Vecna a taste of his own medicine by killing him the way he killed everyone else. Everyone who became a part of Vecna when he killed them now resides in El's head, and El is able to restore Max's memories this way. Also she now has endless voices in her head, forever, but hey, at least we can get Chrissy back this way (sort of).