Warning: this article contains spoilers for season two of Stranger Things.

In the final episode of Stranger Things 2, as Eleven, Mike and the rest of the gang slow-dance under a twinkling glitterball at the end-of-term Snow Ball, it seems the series is about to be wrapped up with a big shiny happy bow. But the rug is pulled from under this upbeat conclusion as the Mind Flayer mega-demon is revealed to be watching from the Upside Down.

This is a more ambivalent ending than that of Stranger Things season one, when Eleven (Millie Bobby Brown) sacrifices herself (or so it appears) to save her friends. Second time out, the citizens of Hawkins have been lulled into thinking all is well, even as their inter-dimensional foe slowly draws its plans against them.

Series three has not yet been officially green-lit by Netflix. However, Stranger Things creators the Duffer brothers have confirmed they are already working on the third instalment of their Eighties-set sensation, with at least two more to follow after that.

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With this year's run of episodes having essentially served up comfort-food reheatings of the original Stranger Things, for how long can the Duffers continue to push our nostalgia buttons? Might the reference-packed retro formula start to grow stale? Some will claim the law of diminishing returns is already kicking in.

There's an argument that with the new season having arrived less than a week ago - and many casual fans still making their way through the nine episodes - it's too soon to think ahead. On the other hand, with a year at least until part three, it's not as if Stranger Things devotees have anything else to occupy their time. So here are the big questions the Netflix hit will need to answer.

1. Is the Mind Flayer coming for Eleven?

When conceptualising season two, the Duffers had the Snow Ball up on the whiteboard as the final scene towards which they were building. Working backward from that made filling in the blanks easier. Only at the very end was it decided to tack on the additional shot of the Shadow Demon looming above.

"We don't end it on a totally happy note, do we?," Ross Duffer said in an interview with the Hollywood Reporter. "There were discussions about that, but then we went, 'Nah, we have to hint at what's to come.' They've shut the door on the Mind Flayer, but not only is it still there in the Upside Down, it's very much aware of the kids, and particularly Eleven. It had not encountered her and her powers until that final episode. Now, it knows that she's out there."

2. Will we learn more about Kali - AKA Number Eight?

Removing Eleven from Hawkins and putting her in a cheesy superhero movie for an entire episode was highly divisive among Stranger Things viewers (actually it wasn't all that divisive - everyone apart from the Duffers loathed episode seven).

In addition to testing the loyalty of even the most hardcore fans, The Lost Sister confirms Eleven is not the only Hawkins Lab escapee with superpowers. Though the Duffers have defended the detour on the basis that it helps bring closure to Eleven's origin story, they also are clearly laying the groundwork for an expanded Stranger Things universe. Will they pick up this thread in season three - or has the fan and critic backlash convinced them to never, ever leave Hawkins again?

3. Can Steve carry on without Nancy?

If any character can be said to have had a satisfying arc on Stranger Things, it's one-time boorish boyfriend Steve (Joe Keery), who has had his heart figuratively ripped out by smart-girl Nancy (rather than the other way around - as it is supposed to go in Eighties movies). Though his meme-worthy quiff is intact, Steve has otherwise changed utterly and finished Stranger Things 2 as big-brother mentor to the younger kids.

As we were reminded when he begs Nancy (Natalia Dyer) earlier on to help fix his terrible college admission essay, it's the end of high school for the tall-haired bro. But he's talked about skipping college and taking a job in Hawkins - tragic for Steve, but the perfect excuse to keep him in the show.

What, in the meantime, about Nancy's new boyfriend Jonathan? Yorkshire actor Charlie Heaton was denied entry to the US - forcing him to miss the Stranger Things premiere - after customs officials found trace amounts of cocaine in his luggage. He has not been arrested and no charges have been filed, though it remains to be seen how this might complicate his future involvement.

4. Will we learn more about the mythology of the Upside Down?

The hellish otherworld is the black hole around which everything else in Stranger Things orbits (and into which it is invariably pulled). But, aside from serving as breeding ground for petal-headed monsters, the specifics of the Upside Down remain deeply mysteries.

A detailed explanation would probably ruin the magic - nonetheless, a fuller understanding of this negative-image Narnia would give a clearer sense of the terrors looming over Hawkins. The Duffers skirt around the particulars of the Upside Down in season two - leaving lots of room for series three to fill in the blanks. In particular, it would be instructive to know whether the Demodogs of Stranger Things 2 represent the Demogorgon earlier in its lifecycle - or if they are an entirely new genus of extra-planar nasty.

5. What new references will Stranger Things 3 squeeze in?

Because the young cast are visibly growing up on screen, the Duffers can't cheat and pretend no time has passed between seasons.

Therefore, when series three arrives a year or so from now, a year or so will also have passed in Hawkins. It will be 1985 - meaning the siblings can cram in references to Back to the Future (Michael J Fox's "life preserver" jacket will inevitably receive a nod), Fright Night, Cocoon, Mad Max Beyond Thunderdome etc (though hopefully not Goonies, to which it has alluded constantly anyway).

6. How will Stranger Things handle adolescence?

The charm of the Stranger Things kids is that they're smart, somewhat cynical and increasingly sweary - but still wide-eyed and childlike. As they head into their teens, though, might some of the gloss come off?

Do we really want to see Dustin gagging on his first cigarette or watch Eleven and Mike get blathered behind the bike-shed on the contents of a raided drinks cabinet? The icky side of adolescence doesn't feel very Stranger Things. It will be fascinating to observe how the Duffers deal with their characters's growing pains.

7. Is Brenner coming back?

Eleven's "death" in season one is portrayed with enough ambivalence to allow the Duffers resurrect the character without it feeling like a cheat. The same cannot be said for the evil "Papa" of Hawkins Lab (Matthew Modine), shown being bloodily cut down by the Demogorgon. So it's a genuine shock as, towards the end of series two, it is claimed Brenner is alive.

Admittedly this nugget emerges in the course of the awful "Eleven-joins-bargain-bin Justice League" episode - which many of us have already purged from our memory banks. On the other hand, if this is a legitimate hint of what's to come, it will be interesting to see how logic can be contorted to make Papa's return appear plausible.

8. What will the show do with Max and Billy?

The dysfunctional new kids in town have not been universally welcomed by fans. Max (Sadie Sink) is an agreeable, if thoroughly nonessential addition to the gang - but Billy's angry bully storyline is in the end confirmed as mere padding.

His raison d'être, it appears, is to antagonise Steve and ultimately brawl with him - and while their slug-out is bruising, it hardly justifies Billy's extensive screen-time. Will series three find a better use for Jon Bon Jovi's evil twin?

9. Will Stranger Things get weird again?

It's vaguely implied that Billy (Dacre Montgomery) dislikes Lucas (Caleb McLaughlin) because Max's new boyfriend is African-American. But no further evidence of Billy's racism is offered and having skirted (very ambiguously) around the subject, Stranger Things never goes there again.

Likewise dropped out of the blue is the bizarre flirtation between Billy and Nancy and Mike's mother Karen (Cara Buono). The scene is creepy not due to the risqué tone but because it is so out of synch with the rest of the series, to an almost Twin Peaks-esque degree. Is this merely clumsy writing - or are the Duffers trying to move beyond genre storytelling?

10. Have Dustin and Sheriff Hopper been infected by the Mind Flayer?

Stranger Things makes a point of showing both Hopper (David Harbour) and Dustin (Gaten Matarazzo) sprayed at close quarters with an icky substance while clambering in the Shadow Demon-infected tunnels. On the presumption that, after consecutive kidnappings and alien possessions, Will Byers (Noah Schnapp) will receive a pass next season, might these two beloved characters serve somehow as the Mind Flayer's eyes and ears in Hawkins?