I saw someone suggest that the Mystery Thing would be a Miitomo/Tomodachi Life follow-up, which I think is objectively the most likely outcome.
BUT
I have a dark horse theory. That this will end up being related to Smash Bros, or at least Sakurai-related. It sounds like utter nonsense, but there are a few things that make me humor it.
- Development has almost assuredly started on Smash 6. We know that there are two Bandai Namco studios working on Nintendo exclusives, Ultimate got those new spirits earlier this year, and Sakurai is done with the youtube channel
- Speaking of the Youtube Channel, Sakurai has said that the last "normal" upload will be on the 15th, with a "Special Suprise" at an unspecified later date. Maybe it'll be on whatever the Mystery Thing ends up being?
- Sakurai seems to be genuine friends with Toby Fox - I remember hearing that he gave Sakurai the idea for the Youtube Channel (but I could be misremembering). I think it's possible that Sakurai ends up taking a few more ideas from Toby. Maybe, similar to Deltarune Chapter 1, he unveils a new project with little more than cryptic social media posts?
I'm not denying that this is extremely unlikely, and probably the Smash Brainworms talking. But there's a slim chance, and I want to be able to brag about getting it right if that slim chance ends up being correct.