XVI'll probably dominate alongside VIIR and likely a small update on Forspoken. I'm really looking forward to more XVI since its world looks super interesting.
As to this potential unknown FF, here's my three utterly out-there speculations; a standalone spin-off (
secondary rumor hints it's a Team Ninja collab with Souls-like aspects); a sequel to another title (X-2, XV-2, Type-1 anyone?) co-created to another dev; or...Haerasis! (that's just silly).
There's even a "cross-gen Eidos project" apparently gonna be revealed. And this isn't counting all the announced stuff they'll probably have trailers and info dumps for outside of FF. Golly, this E3's shaping up to be explosive!