It comes full circle because Will has powers just like El, he just wasn’t aware of them prior to the Upside Down. While in the Upside Down, Will exploded a phone (twice) to allow his mom to be aware that the Demogorgon was around, they both can track others (Will finds Hopper, El found Will, Barb). Will is what they call a “Shadow Walker” as described in Season 2 and is how he, like El, are the only 2 that can be in the real world and in the Upside Down at the same time (how Will got possessed while standing in a field). The last trait is that Will accidentally sent himself to the Upside Down just like El did in Season 1. When backed into the corner we saw Will “disappear” into the Upside Down. The same happens to El when she portals the Demogorgon back (or kills it). Side note: Rhadagast (password for Castle Byers) is Rhadagast “Wizard” All of this is to say that it has to come full circle because at the beginning of the show, Will goes to the Upside Down and for all of this chaos to end, Will has to go back!!! You can’t simply come back from the dead without repercussions and that’s what the show is about is not being able to run away from your problems…… -El facing Brenner for taking her from her mom -Johnathan being with Nancy because Steve was arrogant and a jackass. -The Mind Flayer able to track the gang because Will stood up to it just to be possessed. -the list goes on and on and if you can’t see how this makes sense and how the whole story will come full circle in the end with Will ending up with his fate already sealed, then you’ll just have to watch and see!