There are still a couple of characters whose arcs haven’t been finished:
Applejack doesn’t have an arc, which is a big part of why I find her boring. There’s no clear way to give her one, either–she already has everything she wants, and all of her vices appear in one episode only. If she could acquire one, maybe something to do with the loss of her parents or the family responsibilities that get in the way of her enjoying herself or something like that, it would help the character a lot.
Spike is always either a loyal servant or a greedy jerk; he has no other modes in which to relate to people. And now that Twilight’s need for a voice of reason is decreasing, his position as her servant is making less sense. He’s tried latching onto being others’ servants (Applejack, Rarity), as well as increased the frequency with which he enters greedy jerk mode, but that’s not really growing. There’s potential here for an arc, and it’s never too early to warn kids away from Nice Guy Syndrome.