Ahri is a fairly easy champion to play for mid and is one of the most well rounded mid laners in the game. All you need to know how to do is land her E and you can carry as her pretty effectively. If you have the time, I would also suggest playing Ekko. He's higher skill cap, but can carry just as hard, if not harder, if you know what you're doing. Leblanc is also a really good choice for mid lane.
Fizz is Fizz. He's not as easy to play as the mastery 7 Fizz mains who destroy you make him look, but he is strong when you know what you're doing. For Diana, I would sooner play her jungle than mid. She's got pretty bad matchups in mid since she's melee and doesn't have a gap closer until level 6, so she's easy to bully out (Akali suffers from these same issues). Xerath is, simply put, range. He can burst his opponent from the other side of the lane, but he lacks mobility. Lissandra is a CC machine. She's like Nautilus, but with less tankiness and more damage. She's also incredibly safe, though she's not seen very often and when she is, it's typically top lane.