I waited until the first 5 minutes came out before posting this, but here goes...
Theory 1: Demogorgons & Project MKUltra
I believe that the first generation of Demogorgons were members of Project MKUltra. This is mostly due to the fact that Demogorgon bodies are too coincidentally similar to the human anatomy. I think it was certainly a failed project by Brenner, especially to silence the testees. I think Terry was excluded because she had obviously already been very vocal about the project since even before they could do anything to her. This also supports the popular theory that Dr. Brenner knew about Dimension X well before the 1979 incident between 011 and 001.
Theory 2: Upside Down & the future
Now, this is a very popular theory, but I wanted to expand it a bit. The basic theory is that the Upside Down is what will happen to Hawkins/the world in the future. My theory expands to the idea that Dimension X is an even further future state of the planet as a whole, and whatever happened in 1983 caused a paradox that created a dimension which replicates the earlier future of what the world would become, but limited to Hawkins for a reason to be told. Now this theory dictates that Season 5's plot will not only be killing Vecna, but also stopping Vecna from causing the very event that will lead to the Earth's state becoming that of the Upside Down and, eventually, Dimension X. So, essentially, Vecna would be trying to cause an incident that leads to a future which is also the past that led to his creation & power.
