FFI: Origins release, too many remakes/enhanced ports with gameplay changes to count
FFII: same as FFI
FFIII: complete 3D remake with a completely changed story, PC release
FFIV: complete 3D remake + sequel, Chronicles release, PC release
FFV: anime sequel + enhanced mobile port, Anthology release
FFVI: enhanced mobile port, Anthology release
FFVII: film sequel, tps sequel, two prequels, anime OVA, PC release + international version
FFVIII: PC release + Chocobo World
FFX: sequel + 2 HD remasters + International Version
FFXI: various expansion packs with continued support + PC release
FFXII: sequel + International Version
FFXIII: two sequels + PC release + International version
FFXIV: sequel + expansions with continued support + PC release
FFXV: already talks of sequels and a PC release
So why do you suppose FFIX has never gotten any sort of expanded treatment? It sold very well and was extremely well received, to this day remaining the highest rated FF title on Metacritic, yet nearly fifteen years later there's no sequels, no prequels, no ports to other platforms, much less PC, no remake or remaster or International re-release of any sort, nothing.
Perhaps it's because it was the last FF to come out before Squaresoft/Square Enix adopted the "milk these games for every dime they're worth" mentality?