Switch 2 would require more optimization. On PC they're mainly targeting DX12U and Windows APIs through the GDK...which are natively compatible with Xbox Series. So I don't see why an Xbox version wouldn't happen at this point when it'd require less work than a Switch 2 version, even if the Switch 2 version would sell a ton more copies.
Basically, yes a Switch 2 version is likely definitely going to happen but not before an Xbox Series X & S version, the way things look to be going.
In any case, it's just kind of a realization on my part that SIE really have almost no genuine massive 3P exclusives going forward, even from the big Japanese publishers, and any future FF exclusives will probably be short timed exclusivity or no exclusivity most likely. But SIE will probably still pay up for marketing rights.
Rather unfortunate all around unless SIE are working with SE on exclusive revivals for stuff like Vagrant Story or Parasite Eve, or are doing internal or 3P co-op revivals for 1P IP like Legend of Dragoon, Arc The Lad and Dark Cloud. Otherwise they are just losing more 3P exclusives as content differentiations which naturally weakens the total appeal of the console over time.
I think Nintendo should be very perceptive about FF16 coming to their new console, as the franchise itself was born in Nintendo consoles and it would be a nice comeback to signal Nintendo extending their hand to 3rd party developers.
However, this is what I think Nintendo should do, not what I think Nintendo will do. They seem to not give a rat's ass about 3rd party developers.
Just look at what Nintendo just did. They casually delayed the Switch 2 by half a year because they thought they didn't have enough of their own games ready on time. And this meant that all the 3rd parties making games on time for the Switch 2's release date are going to lose on all holidays sales numbers, not to mention being starved of revenue for months.
Nintendo have gone out of their way for Square-Enix in the past, that's how the Crystal Chronicles series came about on Gamecube. They've also got a strong relationship with SEGA for various Mario & Sonic crossover games, appearances, and the Bayonetta IP license.
So I don't think Nintendo reaching out to Square-Enix to do as you suggest, is far-fetched, considering they've done similar with even Western publishers the past few years like Ubisoft.