Mmmm... let's put some events on a timeline. Please take into account that all the stuff we have no sources on is entirely my speculation, so PLEASE don't quote me on this or add it to Wikipedia.
On
October 18, 2021, the last Ultimate DLC character, Sora, was released. I THINK that around this time, Nintendo started getting into talks with Sakurai for the next game. Nothing official yet, but since Switch 2 was likely meant to release earlier (before Nintendo realized they could a whole extra year of OG Switch consoles), it's not far-fetched to think Nintendo was likely planning on Smash 6 as early as here.
On
December 1st, 2021, the series got its final balance patch, and it will not get another version update until over two years later (which I'll get to later). This seems to indicate that not much dev work went into Ultimate through 2022 and 2023, so I think it's very likely that during these two years, Nintendo made the necessary agreements with Sakurai to get him back as Director for Smash 6. I think it's possible Sakurai spent these two years working on early planning for Smash 6, coming up with new ideas and characters, while still working on the final updates for Ultimate along with his video series.
On
November 13, 2023, it was announced that
Namco Bandai has created two studios dedicated exclusively to working with Nintendo. This very likely means that NB will be returning as the dev team for Smash 6, which should make development of Smash 6 a lot smoother (since according to Sakurai himself, using the same dev company and team means they can reuse assets from the previous games they developed). I think it's probable that around this time, the dev team was fully settled and Sakurai pitched his idea for Smash 6 to Nintendo and development of Smash 6 officially started, with Sakurai still working on the last update for Ultimate and the last videos of his Youtube series (which is ending soon as of this post).
On
December 6, 2023, the final Spirit Event for the game's 5th anniversary is announced, which will release periodically until
March 19, 2024.
On
February 14, 2024, the last Version update of Ultimate (13.0.2) is released, which is the addition of compatibility of the Sora amiibo. With this update, Sakurai officially announces that development for Smash Ultimate is over. And with his Youtube video series now nearly done in terms of production, it's likely that Sakurai and the dev team moved on to Smash 6 development in full at this point.
I really doubt we'll see a new Smash next year, but if the timeline above is accurate I think it's possible we could see an announcement on mid/late 2025 for an early/mid 2026 release. Maybe no later than Holiday 2026. Nintendo values Smash very highly by now so they will likely want a release for each of their systems as early as possible, and I think it's likely they got everyone on board during the final dev periods of Ultimate.