As More Badass pointed out everything on the other side is exactly the same just decayed, alternate timelines wouldn't really make sense. That and the D&D references, vale of shadows make me think it's just a mirror.Plus, whatever new goons the government sends in the wake of Brenner's death would want Will.
My guess is that in season 2, we'll meet actual people from The Upside Down as well as learn more about the Lovecraftian creatures. There's a theory in the main thread that the Upside Down is a world where the Cold War actually happened, hence Eleven being able to travel to that dimension so easily. The monster may or may not have been human at once time, or is an offspring of mutated humans. It certainly is intelligent enough to know when to go back and forth.
I hate to be a bummer but like I said in my previous post thanks to certain shows everyone has been conditioned to look for secrets, they do it in the GOT thread too when I think it's pretty much what you see is what you get.
However the one thing those theory's make me hope is that with a second season and beyond maybe the gate becomes less of a tear and more of an actual gate where each story could be a different place opened on the town.
I do want el back but I think a season two of just looking for her or the same threat would totally ruin the good will this has earned.