The 1st nerf to Riven, Force her to Max Q instead of E 1st...
Why the hell can she max E, and still have enough damage to 100-0 anyone? Her E should feel like a reward factor to her whenever she goes in for trades and successfully block attacks. Not use it to out trade everyone. It's a escape tool and a very strong defensive tool but in most case used to constantly win trades.
Increase the cooldown on her Q Early games, which force Riven to Max it 1st. What this does is, bigger gap between trades if she wants to max her E 1st, more aggressive trades if she max her Q 1st.
Nerf her Passive Damage, and shift its power into her Q.