I feel like the midland offers you a chance to be pretty versatile in terms what champion you want to play. You can go utility mage, something like Heim and offer turrets paired with Rylais for example for the slows in a team fight scenario while easily pushing back a lot of champions. You could also go burst Mage with something like LeBlanc. There's also the wonderful of world of assassins with AD (Zed, Talon) or go AP (Diana, Katarina). I would consider myself a mid main and typically try to counter pick my opponent. For example, versus Heim go Brand, for Zed you can go Heim & harass laning phase while rushing Zhonyas. If you struggle at CS Zed, Ziggs & Heimer are the first that come to mind when easy CSing is the topic. I would suggest basing your champion pool on champions that you're comfortable with in other lanes.
For a new or struggling mid laner I would suggest Lux. She has great range, her snare is great, AoE slow can be a real pain to deal with, shield provides good team oriented ability for later in the game and a global ultimate is a nice way to catch those fleeing enemies. Morgana can offer a similar kit with more wave clear with her AoE.
If you're a more experienced player looking champions that require more practice Zed is a great AD based assassin with good mobility and his Death Mark partnered with Ignite and BotRK can burst down just about anything outside of tanks and players prepared to combat you with Zhonya or something like Wukongs Decoy.
Something AP based that can also provide a challenge would be Brand who is one of my favorite champions in a great place right now. His combos take some practice but are very rewarding. Ahri's kit also has a lot to offer. Her poke is great, her charm is great for initiating in team fights. Her poke can absolutely punish the enemy for being poorly positioned and her ultimate offers great utility whether it be blown to escape or to secure the kill on fleeing opponents.
As I said earlier I would base your selection on what type of champions you play in the other lanes. Don't be afraid to jump into blind pick and experiment there are a lot of champions and I'm certain there are at least two or three mid laners you can pick up and be prepared for most match ups to a decent extent.