Yeah, they most likely have some small core team to know what and how they want XVI to be like, but given that XV's sequels are basically guaranteed, I'm sure it's very early. Also, I don't think Nomura would be involved when it comes to art (I love him, but sometimes... cofcofFFXcofcof). I'd love something more realistic and and mature. Not too much, yeah, it's FF. But if they had a MC along the lines of Cor, Basch, Balthier... and not another pretty teenager, that would be amazing.
It's tricky, though, that it'll be affected by XV's perfomance and critics, so I guess they'll be very careful and maybe slow until they know how to tackle the declining popularity of the brand and also the industry in Japan.
And man, do I miss those times where they basically had completely new entries every few years. That's the main reason I dislike current S-E so much, among other things: I didn't like XII, but had they release a brand new numbered title in a two-four years period, I wouldn't even care about it.