Well, I would not make a FFXV for 10-15 years or until the company is in dire financial straights. It would be an "in case of emergency, break glass" deal.
This will be a complete nostalgia trip for all the people that played all the Final Fantasy's since the NES days. It would be pure fan service, complete with references that only die hard final fantasy players would understand.
Other than modern graphics and an orchestral score, it would be as old school as they come, with random encounters, cutscenes that take 30 minutes to watch, logic puzzles that are explained as you go through them, and grandiose melodramatic characters.
The plot would be you start as resistance to an evil empire and, unlike Western RPG's where you stop after overthrowing said empire, you go all the way and kill God.
It would also have levels: No Materia, no Junction system, no Sphere Grid, no License Board, no Crystarium system. You start at level 7 or 8 and you keep going until you reach level 99. You know, the thing Yahtzee complains about JRPGs having, but never appeared in a proper final fantasy since 9.
The main quest would be 60 hours long, but the side quests would tack on an additional 60 hours.
Characters would have one job and that is it. There will be none of this mage in name only after you break enough locks like in FFX, or classless characters like in FFXII. A character begins life as a thief and remains a thief throughout the entire story. There would be the typical ones: White Mage, Black Mage, Red Mage, Fighter, etc. There would also be some based on specific classes from other Final Fantasy games.
Special features would be a ton of mini games along the lines of you having to play though several minigames that copy the art style, the game play mechanics, and the cliff notes of a previous Final Fantasy game. There would be 6 of these completely optional games, each one completely optional, but would add 6-10 hours of game play to this monster. Each one of these mini games would unlock a series of unique items from the game they were emulating.
The summons would also be fan service. There would be the typical ones like Shiva, Ramuh, and so on, but there would also be new ones that are based on Final Fantasy Characters: You have a 3 sisters knock off that is Yuna, Rikku, and Pain from FFX-2. There would be a 4 Swordsmen deal, where Cloud, Squall, Cyan, and Tidus taking turns slicing through the enemies and it ends with all three of them doing a signature move, Cyan does a quad slice, Cloud does his ommnislash, Squall uses his gunsword and makes one final shot while Tidus does his blitzball strike. Another one might be a needle attack, where Kain and Freya take turns poking holes into the enemy. Hell go nuts with it: Tifa and Sabin take turns beating the ever loving stupid out of an opponent, Lock and Zidane come in and steal something.
So yeah, FFXV would be complete Fan service.