Not quite actually, Square Enix has another division that can work on FF16 separate from the FF7R team (not all of the top FF devs are on FF7R). Rumors that float around is that the division behind FF14 is working on FF16 on the side. I personally think this is a good thing as the team behind FF14 knows what they're doing (a good number of them worked on FFT, FF11, FF12, and I think even FF9).We’re gonna be doing the FFVII Remake up through 2030 at least, no FFXVI is in sight.
But even if Final Fantasy XVI is the latest Final Fantasy game by the time SoulCalibur VII comes out, I still think that it's going to be Cloud that will be chosen as the guest character since he's a major video game icon and Final Fantasy VII Remake parts are likely to still be in development (it will probably be renamed into Final Fantasy Versus VII lol). Personally I would like to see a Dragoon if the guest is from Final Fantasy but the iconic Dragoons aren't popular enough (Kain, Estinien, etc.) in my opinion and I doubt the main character of FF16 will even be a Dragoon.
On a side note, with all the talk about guest characters, I would be pretty upset if we get Jon Snow as a guest character on a potential SoulCalibur VI Season 3 because of dumb fan requests. I swear the ending of Game of Thrones still left a bitter taste in my mouth and I personally think he's not a good fit.
My two favorite guest characters that appeared in the series so far are 2B because I love NieR: Automata and Link because he got me into SoulCalibur but the best fit for the series is definitely Ezio and Geralt. These two fit in SoulCalibur almost seamlessly, Ezio more so.