A year to polish, add in content to the nice world they've created with new quests and things to do, change up the last few chapters instead of having them be a rush towards the credits, playable Insomnia, etc. Add in more cutscenes etc.

The game was like 3 years in production, Open world rpg's from Bethesda and Bioware take 5 years to make, FFXV would've benefitted from an additional year.

Honestly, I am just happy they released the game and can move on to new stuff now. It might've been only 3 years in actual production under the "FFXV" title, but to almost everyone it has been a decade since the concept was originally announced for the PS3. 

I want to see what is next for the Final Fantasy series; it feels as if it is finally freed from a purgatory of obligations. Years ago when they released the Agni's Philosophy tech demo I thought it was a compelling concept. At the time I thought it would've made a great Final Fantasy XV.  Now I am glad that they didn't announce it as a game. I want to be surprised with something new and I don't want them to spend ten years releasing that something new. 

Every big game that's not excellent would get a quality bump with an additional full year of development, obviously.

Well chapter 13 would still have sucked, they are only changing it because everyone complained. I think it would have scored far higher with another year of development, maybe 90.....maybe xD

Probably not.I havent finished the game, and probably wont for a long while, but the game was flawed at its very core.One year extra of development would have fixed some things, but would never make it exactly a great game.Especially now with better games releasing.

If it is any indication, it was better for the game to have released when it did instead of trying it now.Too much competition.

The only major thing that an additional year would have enabled is probably adding more diverse sidequests. Well, other things too, but that's the only thing that would have significantly affected reviews. Adding more things to do to the later chapters would probably have taken more than a year, and the other things mentioned here probably wouldn't have affected the score much. If I had to guess, it would have needed another 18-24 months to really make a difference.

And if it released 7 years earlier it would be top rated game of all time :P.

All the devlopment post release has probably been slowed, compared to pre-release. They are definitely not using the full development team on the updates. So it wouldn't have taken a full year, by far.
However they also used feedback to improve the game, which is not possible pre-release, not on the full game and on a wide scale at least.
It will however influence a lot on their future games, especially mainline FF titles. This was their first game of this scale and they learned a lot from it.

Nah, they need at least two years.

They are using 70% of the original team apparently.