That "single feature," combat, is what you'll be doing around 90% of your game time.   Everything surrounding it, such as the license board, how you gain money, equipment, gambit system, etc, add to that, so it's not just one problem the game has.  Several things surrounding the combat make it worse, but the combat itself is garbage.  I've detailed the reasons I dislike it in other threads so no need to walk through it on here.  

It looks and runs great on PCSX2...

The main thing I'd hope they fix is pacing. There's sections where you go hours without dialogue. The combat sections has no quips etc about what they're seeing like KoToR or Dragon age. It felt too divided between combat and story sections. But, they wouldn't really change anything besides textures. The other question would be would they do the zodiac job system version or the original. The zodiac job system is more interesting but not polished because such a small team reworked the game.

There's also no one saying that the people working on FFXV or FFXVI have to be the ones working on HD ports. They could just assemble a team that's good for that to do it. It's just mostly retexturing.

I'm sure they'll add some tweaks and additions to the battle system.