Riven's kit overall is still just to good. The fact she is a high damage champion with mobility and a shield that scales of AD with a low cd still makes her a tough champion to deal with. If they really wanted to make Riven a lower tier champion I feel a longer CD on her E would achieve that as the last nerf which just reduced the duration the shield last doesnt really affect anything because if used right will still absorb a large chunk of damage and be off CD in a few seconds to be used again.