Since the mindflayer can only be defeated by a undead army or an army of artificial beings (neither of which can be possessed), I think Kali and/or the dead demodogs may come into the picture, especially the one in Joyces fridge. OR i think the dead demodogs may be possessed by the cloud thing that left Will.

Im actually beginning to think that maybe Hopper had something to do with his daughters potential death. In the car with Eleven in the final episode he says he's a black hole and appears to blame himself for his daughters death. This will get explored a lot more and may potentially even lead to Hopper being reunited with his actual daughter.

I think Terry didn't actually meet 11 in the rainbow room but some other kid. The child in the rainbow room is dirty blonde, while eleven is brunette. Hoppers kid and wife, however, are blonde. Terry is a brunette too. I think she just straight up found Hoppers daughter.

I think we'll understand more about why Max and Billy are in Indiana and see if this ties back into the missing kid from San Diego that Eleven saw a clipping of in Terry's house.

I think Will possesses some abilities like 11 at this point and this will get explored in S3 and beyond.

it's possible Lucas, Dustin and Mike will stop hanging out as a group. Lucas will hang out with Max, Dustin will explore his demodog and bring the whole group (and the audience) into an understanding of how the UD works (along with Dr Owens a la Back to the future, which released in this time period) and Mike will be chilling with Eleven and potentially Will. For the first few episodes it'll seem like the AV club is no more until they reunite over something.

I think this "something" will be Dustin accidentally reviving the Demodog. It will escape and turn into a demogorgon, and use it's powers which will attract the shadow monster and cause the shadow monster to possess it. It will open a gate and Season 3 will end at that point.

I don't think Lucas is going to last the series but I've been wrong about Steve before. Lucas is a ranger and if I've been reading my D&D stuff right they're usually the first ones to fall.

There's definitely a reason why we've not seen Kali and her gang promote Stranger Things along with the other kids though they all hung out and know each other. Not sure what that reason is I think the writers may introduce a few other kids from the lab who were sympathetic to Brenner and working with him because they really do think he's their papa.

They've already declared they want S3 to be character driven and not as big as S2.

Billy was pricked by a neddle and develop part of the demongogan fluid that resembles aids

Will will be a main character with visions and connection to the mindflayer.

Johnathan, Will or Steve will die.