Riven is too Resistant to Slows
Her dashes need to be slowed as well as her movement speed while she is under the effect of slows, similar to the way kalista gets slowed. She should be able to negate mistakes after they have happened with her mobility. Sure, mobility can help her avoid mistakes and do things other champs cant do, but save her after the fact? No. If she gets hit by Vel'Coz q she shouldnt be able to dash 300 units away as if she wasnt slowed, so much so that vel coz players wont even waste an ult on her.
I think thats probably the most unbalanced thing on her kit, the fact that its not really worth slowing her because she is immune to slows on a three second cooldown. Problem is, if rot makes these changes she will need to have some of her other pointless nerfs reverted.