At the very end of the Yokohama Arena show (and LEGEND M as well), they showed a number: 20201010, which most interpret as a date, October 10th, 2020.
They also added some text to it:
Beginning in 2010, the odyssey
to the METAL GALAXY continues.
And after 10 years,
XX DAY will be approaching in 2020.
This is seemingly an event for the 10th anniversary of the band. BABYMETAL debuted at Sakura Gakuin’s first show on November 28th, 2010. They played Doki Doki Morning. That isn’t the first activity from them we know of, though. That would be the recording of Doki Doki Morning on October 30th, 2010. That is closer to this anniversary” date.
BABYMETAL does not announce things so far in advance in general. In fact, they are usually the opposite (LEGEND S was announced 1 month and a half before the shows, for example). Though, Wembley and Tokyo Dome were announced 7 and 9 months in advance, respectively. Last time they announced something so far ahead was the anime announced in 2016 that never came to fruition (let’s cross out fingers it’s not the case this time).
I wish there was more information, but that’s what we have for now. Lots of fans think it’s a show, and are already planning on saving for a trip to Japan.