For me the most annoying part of XIII wasn't it's linearity, but its lack of difference in kind.
(See this as explanation:
https://www.youtube.com/watch?v=TlBR1z-ue-I&hd=1 )
All you do in this game is fighting for the absolutely most part of the game.
While this is okay 'cuz it's a J-RPG after all, it was too much.
Running your typical tube-level >> fights >> menus >> cutscene >> repeat
(But I have to say, that I still prefer semi tube-levels instead of gigantic mazes where you waste your time running from one dead end (no treasure) to another - time is valuable to us all)
Sidequests and exploration were at short hand on this game, although XIII-2 did a slightly better job.
I know that they wanted the story to be set one piece and not to be broken by "oh, let's check this cave out for a while". But it was too much. Until chapter 11 the game was truly "meh".
Also: Today people actually don't want to grind to the death. But that's truly what FF is about atm.
Grind, grind, grind...
Oh, and Confusing mechanics like upgrading in XIII (would have never used this optimal without GFAQ, etc.) or "Chaining" in XII (fire the guy, who invented this one SE!) or the time-mechanic in LR (WTF?!).
So... even if XV will be a mechanic-wise mix of XII, XIII and Kingdom Hearts it won't help much if there is no difference in kind at all. The game truly needs stuff like TT or BB as minigame, sidequests, maybe even moral-choices (and at best an ending-slideshow to show the results of your choices.)