Hey all, just wanted to add on a quasi-main to my roster. I main Zed mainly, but I'm looking to work really hard on another mid champion. Any suggestions? I'm not looking for the whole "X is good right now" as that's meta hopping and not maining. Any consistently good mid champs that can dominate well with experience? Looking more for the whole skill floor/ceiling thing rather than cheesing my way to a higher ELO spamming Annie.
and 
come to mind. Neither are uber-meta at the moment, but, both are rewarding if you sink the time into them.
Syndra, Orianna, or Ahri.
They've always been great stable champs.
If you like Zed im assuming you would be a fan of mid Ekko. lots of mobility and funky positioning with the ult, similar to the shadows.
Actually very dangerous in lane if you can sustain with your passive and land skillshots; at level 3, an orb stun into a combo hits most mid laners for ~40% of their health, more like ~60% at level 6. Kill pressure is low pre-6, but you can force people out of lane over and over and just crash waves into their tower for free.
In any disadvantageous matchup (rip gap closers), you can basically farm from tower with spells alone and look for roam opportunities. The easy farm is great for 
's clean build path, which solves any of your mana issues and typically lets you one-shot waves with Q-W for heavy farming or lane pressure. Xerath also spikes quite hard with 
, which helps him kite and makes his poke/burst devastating.
Basically, you can come out ahead or neutral in 90% of lane matchups, have semi-global kill pressure with your ult, and huge sidelane pressure with your efficient waveclear, which collectively allow you to carry the midgame. Lategame, you scale into global artillery that can enter a teamfight by using ult to lock down the enemy backline, before moving in with low-cooldown AoE nukes and potential 2s stuns.
