What if we didn't nerf mobility
Instead, we create a champion pretty much specifically designed to fight against mobile characters.
For one, an ability could be something among the lines of "Deals X damage after Y seconds, amplified up to 3X the amount of distance traveled by the target over Y seconds.
Of course, this is just eyeballing something and definitely could be the active of some kind of tank item, which would be helpful to tanks.
Anyways, thanks for listening to my short little rant, it could probably have been longer and more in depth if I had the time and was willing to try and keep it from dying, but I'll just leave it at that.