We were also told in an interview that Emet and the rest of the ancients stuff in EW was already planned out from the start and had nothing to do with his popularity. The plot with them would have probably stayed the same regardless of whether or not we got a garlemald expansion. Too many plot points about the ancients in EW were laid down in 5.0-5.3 in ShB to feel like they were shoved in for fanservice. Also they planned condensing expansions pretty early into 5.0, if there was a plan it probably never made it past a story board. Not saying garlemald didn't get shafted though.
They pretty much said this in a Famitsu interview with Yoshida and Ishikawa back in 2022, yeah. The broad strokes were already storyboarded in 2019, in the months after Shadowbringers came out (because the CGI department needed the storyboards to start working on the big cinematics), and the storyline would have stayed about the same, just longer - an "end of Garlemald" expansion culminating with a fight against Anima, and then a big finale expansion with Zodiark, Hydaelyn and the Twitter Bird.
But they saw the fan response to Shadowbringers and decided to capitalize on the momentum by canning the Garlemald expansion and going right to the finale.
Final Fantasy X is the sole mainline game that hasn't been thoroughly mined out yet. The only references it got so far was a paid Tidus costume (might be others as well), and a small Thunder Plains reference in the Eden raids.
There's also the Ixion FATE in Gyr Abania, and the mount you get from it.
And DT has both the Pelupelu and their beast tribe questline, which is pretty much one big reference to Rin's Travel Agency in X.
TBH I suspect that we would have seen more FFX references in the notional Garlemald expansion, if it had not been canned. As it is I think they've been put back behind the glass in case of emergency, the same way VII seems to be, and occasionally they'll pull something out like the Pelupelu.
OTOH I think VII might also just be reserved because CBU1 is still doing the Remake stuff, and they don't want to step on each other's toes.
That really just leaves XIII, which... tbh I have no real idea about that. Never played it, don't know much about it. I am familiar with Lightning and that's about it.
There's also the newer ones, but...
XV got a crossover event, and I suspect that's all it's likely to get. It's too new and none of the antagonists etc really have any of the "hey, remember this?" nostalgiabait factor you can count on when mining stuff from the older FFs for theme park stops.
So did XVI for that matter, and that one is probably also going to be left at that, for broadly the same reasons but also like... Valisthea is just kind of Eorzea 2, by the same people. They could just have done that in XIV if they wanted.