A unique, highly customizable game where players would need to depend on one another's skills would be the best for me.
I just registered beta testing a game called Furvilla and that game nailed it so far with the careers heavily relying on other careers (e.g. a tailor needs fabric from a herbalist and accessories from the jeweler, who'd also need seeds from explorers and parts from warriors respectively). I'd love to see something like that.
Also your ability to create multiple ponies or maybe your own town like in PonyIsland, that'd be so cool!