The solution to mobility creep is in the game already. We just need to change a few interactions.
- A champion's dash is now considered to be at 100% effectiveness when their current movement speed matches their default movement speed. When their current movement speed is diminished, their dash range is diminished by an equal amount.
What this means: Slows will actually slow champs, since nearly everybody can just dash over a slow field now. For point and click abilities, a champ with a (say) 500 range leap will, with a 25% slow, have a 375 range leap for the duration of the slow effect. This affects non-targeted blinks and Flash as well.
Does not affect champions whose current ms is higher (though that would be an interesting ability for an item or a support).
If Zed does not currently have a range limit to his shadow hopping, he should, and then this can affect him too. Slowing him to keep him from popping away is counterplay, and he could use some.
- Roots (snares, immobilizes, etc) will prevent champions from using movement abilities already. What they should do is stop them dead upon application as well.
What this means: Roots will stop a champion from just jumping onto you anyway and killing you while they're "immobilized" next to you. Jumping animations (Tristana, Jax) will drop to the ground immediately at a sensible point along their flight path (where they would be if it was just a dash towards their destination).
Bullshit like Fizz's dash doing damage when he launches and not when he hits you will also be fixed. When a champion stops, if they are not within autoattack range they do not deal damage.
Ultimates that specifically prevent this (Vi) will be unaffected.
For the reply:
When you howl into the night sky that your OP as fuck main will be ruined forever by these changes, consider a list of champions who will be "ruined" in this way, and then consider the health of a game with that list of champions getting nerfs they sorely need. I think you will get over it. This is a better solution to the very real problem of mobility creep than reworking every champ who abuses their mobility (which is nearly all of them). A few of them will still need additional work, of course. You know who they are. ~Uiru
Ezreal E
Kassadin R and Flash there might be some more I can't think of (these abilities already have the advantage of being able to go over
Azir R wall and Veigar E (stun cage) without being stunned

on ranged champs would be OP. Bad idea
It's just a design problem. Mobility has to be present at a certain cost/giving a certain weaknesses. Find the champ that gets more than what they should along their mobility and subtract something, then the problem is solved
being the main one that comes to mind. Though they removed it a little while back.