It's annoying that they did direct sequels for FFX and FFXIII, neither of which really needed them but didn't do a FFXV-2, which could've been pretty damn good.
I'd have loved a second game set in the same world that added all of the areas that they didn't get round to finishing in the original. Essentially turning the entire world of Eos into an open-world environment. So you could be driving the streets of Insomnia and take off if your car-airship and then fly all the way over to Tenebrae. I'd absolutely love if they'd managed that. It feels like that's what they wanted originally.