Give him a 17% movement speed increase.
Not only can he keep most marksman at arms reach due to his range, this makes it even more easy.
Solution: Make Aether wisps movement speed unique and 7% on all items. Job Done.
Ben Sharpio1 votesbut muh TF wont be able to function
LatetotheRace1 votesor, OR, he could just be fine overall. 
and that keystone are more than enough from what I've seen.
SrbLud1 votesI'll try and make it more complete.
ADCs aren't that strong in early game to deal with him. They depend on items too much. You need a 3 items while he just needs one ( morellos ) to be relevant.
ADCs are good for sieging and poking. Ziggz can do it better and earlier with a lot of scalling and good cd.
He can poke you to oblivion cause of low mana cost and range, push you in and siege your tower before you get your second item.
You want some help from your jungler? He is awesome at evading ganks with the quick w and flash. Combine it with some cc from his support ( his support is guaranteed to have cc, they work well together ). He is a beast early and scales well into mid game if he gets lich bane second.
Hyperiön1 votesThere are only 2 AP items that normal mid laners would buy. the ones you mentioned.
Meanwhile, Zeal builds into 4 items.
LatetotheRace1 votesI'm aware of that, and quite honestly I think Zeal needs similar adjustment. That being said, do burst/artillery mages really need extra movement speed? It seems a little unnecessary for most champions to have access to it.
Hyperiön1 votesWell its the same boat for ADC's too. Most of them are immobile. Same with most mages. Aside from flash.
The little MS is nice to have so you don't get chased to death.
LatetotheRace1 votesThe MS only makes it harder to balance the champions around their inherent weaknesses. When you minimize a champions weakness all you do is amplify their strengths, and it's painfully apparent how hard that is to deal with.
Hyperiön1 votesTBH, they won't ever get balanced.
Lauchmelder1 votesIt has something to do with safe waveclear and him being an insane siege champion, not because he gets 17% higher MS from 2 items. Also, you know why he's better than current marksmen? Because he hits his powerspike a lot earlier than a marksman.
LatetotheRace1 votesIf he had a very low movement speed (without as much bonus%) marksmen would get to him, and he would be unable to survive the all-in, nor would he be able to escape.