Square Enix has announced that it will hold a Nier 15th anniversary livestream in April, and that the stream will feature appearances from key franchise figures.
Per the description for the stream on YouTube (for which we're using machine translation), the presentation will last for around two and a half hours, and will feature "the latest information on events and merchandise" commemorating the franchise's 15th anniversary.
Additionally, developers will hold "talks", and a "mini live performance" will be featured as well, with pianist Shotaro Seo and guitarist Takanori Goto seemingly set to perform, although we don't know what they'll be performing.

Naturally, the question on every Nier fan's lips is whether or not a new game will be announced during the stream. Square Enix hasn't given any indication that this is the case, but the franchise's 15th anniversary seems to me to be the ideal time to reveal a new game, assuming such a thing is in the works.
We're also nearing (or perhaps that should be Niering) the eighth anniversary of Square Enix and PlatinumGames' Nier: Automata, which remains the last major tentpole franchise release, discounting 2021's Nier Replicant remaster.
Nier: Automata has proven to be a solid little seller for Square Enix; the game has racked up over nine million sales since launching back in 2017, although whether that's enough to match Square Enix's notoriously high targets is anyone's guess.
As well as Automata and Replicant, the Nier franchise also got itself a mobile action RPG in the form of Nier Reincarnation, which launched in 2021 and ran for a few years before shutting down in April last year.

Two and a half hours is a mighty long time, but it's entirely possible Square Enix intends to use that time simply having Nier personnel reminisce about the franchise, which still wouldn't be a bad way to spend the stream.
In any case, the Nier 15th anniversary livestream is set to air on April 19th at 6pm Japanese time, which works out at around 5am Eastern or 2am Pacific. Looks like you're going to have to wake up early for this one.