1 - World map, like the games had before FFX. Add underworld levels as big as the surface world with towns dungeons and new sets of monsters. And add upper world levels. Sky? Why not space somehow?
2 - Bring back something like the Gold Saucer. Put all the little mini games here. I liked FFX's blitzball. Was a fun concept. The card game system in FFVIII was fun, except for the crappy way rules passed on to everywhere. So yes, feel free to make blitz and the card game come back as mere attractions on the new Gold Saucer. Don't force them on people. That's the trick to make mini games work on games like FF. Did Square learn nothing with FFVII? People like the Saucer for a reason. And you can get neat prizes you can use in the main game.
3 - Bring back FFVII's chocobo breeding system. Add a platinum chocobo after the gold. That chocobo could fly and reach the upper levels or the world map. Chocobo races on the Gold Saucer again obviously are a must. And hey, that together with games like blitzball and the card games could be played online with someone else if we wanted to. See Square? Multiplayer implemented that doesn't make the game suck. How about that. Who knew it could be done by being just optional. You know, to have fun with someone else if you got tired of palying the AI in the Saucer. And no. No better prizes if you did and no trophies about multiplayer. Just good clean human interaction fun.
4 - Bring back either FFVII's materia or FFVIII's junction system. Or both. If both, make them completely optional. Optional as in: Do you want to use them both at the same time? Do you want to use FFVII's system only? FFVIII's only?
5 - Make your summoned monsters customisable. Down to their last strand of hair. If the materia system was implemented the could even be equipped with it and depending on their elements and whatnot they'd have different abilities when you used them so we could see lots of neat new animations, keeping the battles a little fresher.
6 - Make your characters customisable. In terms of what they're wearing, if I equip a new set of armour I want my character to wear it in-game, in video sequences and in battle. The same goes for new weapons I equip. I want to SEE it all. Accessories, hats, boots, gloves, everything.
7 - Focus on the writing. Hire writers. Not computer engineers who read a couple of books and think they can write. Proper writers.
8 - Vehicles we can control on the world maps. I want to control the ship, like in every Final Fantasy before FFX.
9 - Make the general visual theme more like it was on FFVII. As in, steel, machines, cold industry. I'm tired of the over-worked textures everywhere. They look good, but if you keep using that over the top artsy crap it gets tiresome and we could use a change in pace from that. The human brain needs change. Tell Yusuke Naora to get his act together, bloody hell.
10 - Hire Nobuo Uematsu.
11 - Turned based battle system. 3 characters. Or 4. Or 5. I don't care, just bring it back.
12 - Don't make the main character feel less important than the other ones, but still make them all feel important and don't leave them on the background. FFXII and FFXIII were crap on this aspect. Don't know about FFXIII-2 because honestly I gave up on FF.
13 - I liked how in FFVII we had an evil corporation that we could fight AND we had the bad guy as well.
If this stuff doesn't fix the series, I dunno what will.