I'm seeing rumors in the Arab games journalism scene that the mystery game is a (now former) Xbox exclusive that's not out yet. It's supposedly an Action title too.
Tweet translates to:
According to the media figure Omar Al-Amoudi:
"The Xbox exclusive that will be announced for PlayStation is an action game that has not yet been released on Xbox devices."
The rumor is coming from this Twitter space that he had a few hours ago (a bit hard to parse what he's saying because he's having a lot of little mic cut outs while he's talking). Go to the 7:30 mark.
x.com
I'm still listening so might get more info out of it, and will x-post to other threads if needed.
------
With this info, my money is now firmly on
Indiana Jones, it's confirmed showing up in Gamescom, it's an action game that's slated for Xbox consoles, but isn't out yet (which excludes Starfield). This is the game that fits the most in my opinion.
If it's indeed Indiana Jones, then that could be a massive L for Phil Spencer, because he paid for full exclusivity of the title (as revealed during the FTC trial), so if the game comes out on PS5 around launch time, then that is basically money and effort wasted for no actual gain.