I'd really like to see;
1. A Fluttercord episode
2. The Main Six's parents
3. The return of Cheese Sandwich
4. The return of The Great and Apologetic Trixie
5. More about Equestrian history
6. More of Equestria and the pony world in general. We haven't travelled much in the show, have we? In 4 seasons we've seen Ponyville, Cloudsdale, Canterlot, The Crystal Empire, Rainbow Falls, Manehattan and the Everfree forest.
7. More Luna. I wish the Luna in the show was more like Comic Book Luna. She's awesome.
8. More side-characters/background characters. Derpy, Vinyl, Lyra, so on.