What new product is OnePlus teasing on Twitter? (…and no, it’s not the OnePlus 8)

OnePlus has made a cryptic announcement on Twitter, and we’re trying our best to decipher what it could possibly mean. Here’s all we know so far.
UPDATE: The riddle has been answered! OnePlus has created a piano from 17 OnePlus 7T Pro smartphones, which will tour the UK, France, Germany, and Finland. Read on to see how the mystery unfolded…
And here it is, the #OnePlusPiano! Created out of 17 #OnePlus7TPro devices, this instrument will make the music you love sound Fast and Smooth. ?
We're kicking off in London on Sunday, and will tour across France, Germany and Finland next. https://t.co/Lxdm28l1pF pic.twitter.com/lph5ESl8Bd
— OnePlus UK (@OnePlus_UK) November 8, 2019
The tweet embedded below appeared on OnePlus’ official UK twitter feed, without further explanation. It seems to show several phones lined up next to each other, in almost complete darkness. It’s very hard to make out what there is. A follow-up tweet still didn’t clarify much: OnePlus only said that nobody had yet guessed it, but that “it’s not a new phone”. This leaves us with far more questions than answers…
No one's worked it out yet! And no, it's not a new phone…?
— OnePlus UK (@OnePlus_UK) November 7, 2019
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One canny user tried brightening the photo to see if there were any hidden clues – but that just revealed text saying “nice try”.
"nice try" ? pic.twitter.com/x2EmENVLE1
— Rico (@XperiaArts) November 7, 2019
The closest guess so far was made by @shen, who suggested a “OnePlus Desk”. The response?
Not quite! Getting warmer though…?
— OnePlus UK (@OnePlus_UK) November 7, 2019
As of now OnePlus’ new product remains a mystery, but that just makes us more excited to see what is the biggest product launch from the brand since the 7T series of smartphones.
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We were very impressed with the OnePlus 7T, which we described as “an excellent bit of hardware”, awarding it 4.5 stars out of 5. We loved the speedy performance, the speedy charging capabilities, and its super smooth 90Hz AMOLED screen. However, we reckon the camera still needs an upgrade before it can compete with the best on the market.
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We were also impressed with the OnePlus 7T Pro, even though it was only a minor spec bump over the OnePlus 7 Pro. Nonetheless it’s a great phone for a good price, with a distinctive pop-up camera design and a combination of high performance and a great screen that are an excellent fit for keen mobile gamers.