At the moment, the closest thing League has as a counter to mobility are point and click CC spells. These however often hurt immobile champions just a much as they do mobile ones.
Point click aren't any more useful against mobile champions than immobile champions.
Skillshots tend to have double the range of point-click stuff, which forces point click to literally only have an edge when you're targeting a specific opponent in a sea of targets.
Range, defense, and CC also tend to allow targets to survive long enough to do their job very much like mobility.
So without introducing any new forms of CC, what are some spell ideas that could theoretically hurt high mobility champions more than they do immobile ones?
Roots already hurt mobile champions more than immobile ones.
If Riven is hit with a root, she can't cast Borken Wings or Valor, where if Darius is hit with a root - he can still cast everything in his arsenal, hooking an opponent near him, slowing them down, and then dunking their face.
Mobility skills are silenced when you're hit with a root, which is extremely detrimental if they're attached to vital functions (ie Riven's HP shield).
One idea: Reversed Karma tether. Enemy champion is stunned/rooted upon breaking a tether by going out of its range.
That's extremely counter-intuitive.
Viegar's wall essentially does the same thing in a more intuitive fashion.