I got sick during high school, and ended up playing Final Fantasy X on PS2 for more than 200 hours straight, except toilet brakes and sleep. I was already in love with the franchise after playing FFV, FFIX, FFVIII, and FFVII, but even if I wasn't...
Final Fantasy X is simply one of the best games ever made. It has a nearly perfect combat and levelup system, its the most fluid turn-based battle system I know of. The story is amazing and unique. The characters are highly lovable, funny, cool, interesting. The music is extremely good. The only flaw was the English dub. Yes, John DiMaggio is excellent, but still, I would love to replay one of my most favorite games ever with proper Japanese voice acting and English subtitles.
So it must have a separate Japanese option for voice. It really ought to include all three Sphere Grids (alternate level-up schemes) from the various versions, adding an extra GOG Grid would be an amazing flex. I assume high resolution support is a given, since HD Remaster already exists—but it doesn't really need to look "better" than it did on PS2, I think it aged really well as it is. Being able to play with the original graphics as an option would be the best solution.
The only other feature I'd love to see would be support for analog Hall-Effect keyboards. Especially the minigames could use that, and I don't like playing with gamepad on my PC.
So please, GOG, add the final true Final Fantasy to your roster!