It's publicly announced development on a new IP last year, and a few details on what direction they're taking have started to leak out.
"It is (surprise) a triple-A first-person shooter that is still in pretty early development," reporter Jeff Grubb says in his latest GrubbSnax podcast (paywalled). "Looking for 2024 at the earliest, probably 2025." The project's lead departed Respawn earlier this month, but the team is "continuing to prototype."
Grubb says this is Respawn's "strategy. This is how they make games. They prototype them for a few years. A solid block of years they spend figuring out what the game is going to look like. Right now, they have a sort of guidance of 'triple-A, first-person shooter,' and there's a couple of key words: mobility and style."
Perhaps the most notable bit is that this is "entirely supposed to be a single-player game. At least now." Grubb speculates that depending on how the prototypes go, if Respawn finds a particular mechanic would be fun in multiplayer, there could be a multiplayer mode. But the team's focus is on single-player.
The former lead on this unannounced game was Mohammad Alavi, senior narrative designer on Apex Legends, who announced his departure from Respawn earlier this week. Alavi was also senior designer on the Titanfall series, and a designer on Call of Duty. His best-known contribution to that last series is probably the infamous No Russian level.
A Apex Legends.