I hope it actually exists as a real tangible product but I know deep down that 0 work has been done even on a hypothetical game. The reason being , Square is already loaded up with projects and spending tons of money across the board getting all of these games out.
On the online front , they only got FF14 into a good place late 2013 , it's remained there , has so far released one big expansion pack but what's next ? well I imagine they'll keep supporting that title until 2023, 2015 was major update 1 , late 2017 will probably see major update 2 , mid 2019 will see major update 3 and major update 4 will follow in spring 2021 with a final major update near the end of 2023. So if a followup to FFXIV shows up it won't likely launch until near the end of it's lifespan. Probably starts development in 2020 and it's out in 2024, maybe VR based ? who knows exactly. Square has a finite team set up to deal with FFXI/XIV stuff though so you can basically assume whoever is involved with it right now will remain involved for most of it's continued lifespan.
So what's the next major project ? FFXV obviously. People love to throw around the development hell versus XIII suffered but I've said it many times and I think most people out there finally realize it - XV despite it's many similarities and re-used art/design assets is NOT versus XIII. Versus XIII was cancelled in 2012 and from it's ashes rose the FFXV project in 2013, now , in 2016 after a little over 3 years of continuous honest to gosh work - it's finally finished and going through it's final coat of spit shine polish. The rumored september release date coupled with the potential for DLC means at least a small portion of the FFXV team will probably be tied up through to some point in 2017. Where will the rest be heading off to ? ... well -
Kingdom Hearts 3 - The next pipe dream project coming from Square is easily KH3 and once FFXV has finished up and is ready to go worldwide the bulk of the FFXV team will probably be immediately transferred to Nomura's watch to get it out the door in time for next summer. Why next summer ? well when else are they going to release it exactly ? it's the type of game that needs an entire quarter to itself so the latest I see KH3 making it out the door is probably August but July might be better. Thanks to stuff like disney infinity existing , I wouldn't be at all surprised if DLC/crossover ideas existed with KH3 going forward too. Either way, let's say by September of 2017 the 2 biggest square teams are finally free again , what's next ?
Well , right now FF7 remake episode 1 is being built externally but what if , thanks to the groundwork being laid they shifted those 2 big teams to also work on FF7R. This of course assumed the game ends up being no more then 4 episodes. Episode 1 and 4 get farmed out , episode 2 is done with the FFXV team and episode 3 is done by the KH3 team. This ties up these teams for another 3 years BUT if my theory is correct that means the the FFXV team , upon finishing FF7Re2 might just get freed up by the end of 2018. So , perhaps production on FFXVI might start by 2019 and the game itself could be out by the end of mid to late 2022 ... this would mean it's probably going to be a PS5 game obviously. Which also assumes that the PS5 is out by 2019 and that Square is still making enough money from games to bother with another big game. Of course, if FF7R ends up successful with episodes you might see FFXVI take the same route which could mean it'll start trickling out by 2020 with an annual addition. That would make FFXVII the next online installment ....
As for what form I'd want FFXVI to take ? well I don't know man, by the time it's out I'll be nearly 40 years old. I'd love a return to a more steam punk / mage punk style world like what we got with FF6 but fully realized with the power of a modern gaming console (this game would probably look better than agni's philosophy). Give me vast , immersive cities and worlds to explore , an exciting and technical battle system (go back to some form of ATB if it's required) , cool characters that aren't all 16-21 year old pop stars and an overall story that feels like an excellent original idea mixed with elements of nostalgia. Just like FF9 and FF12. What I do NOT want is any attempt at making something at all similar to FF8 / FF10 or FF13.