Xenoblade X: A recent example that any MMO developer could learn things from; both good and bad. I wish FFXIV adopted XbX's design ethos when setting out to make new zones; areas that are not just large, but genuinely fun to explore with something new to always discover, and should take a very lengthy amount of time to see everything along with having a real element of danger added to that exploration. The game also handled it's version of hunts called tyrants significantly better and tied it into the exploration element. tl:dr; a diadem that works :/

Also seamless connected areas something people have always wanted to see. The Wii U only allows 1 gig of ram for use by games but XbX did it so well. The sooner we ditch the PS3 so they can start really making use of the ram in both the PS4 and a modern PC the better.

Phantasy Star (any version): Elemental and enemy type weaknesses/attributes. Someone already touched on this but it would add a nice amount of depth to this game.