Riven nerf suggestion to really hit the core of the problem.
We've seen Riven nerfed time and time again without really affecting what's truly obnoxious about her, and even with the proposed ult changes on PBE, I don't think that's going to make her any less obnoxious. The real problem with her is that her Q is essentially 3 flashes. Even if she's snowballing her lane she's almost impossible to gank because she has 4 dashes in her kit which essentially makes her uncatchable.
I think, instead of hitting her damage, which is kind of her thing, why not hit her mobility? Instead of allowing her to do 3 Q dashes whenever she wants, make it so she has to HIT something with Q to do the next follow up. This adds counterplay to the ability and makes it so she can't use it to escape and gap close as easily. She would have to use her E to get into Q range to ensure that it hits and she wouldn't just be able to QQQ into her lane opponent's face whenever she's dominating her lane. That's honestly the biggest problem with her and what makes her so frustrating to lane against.
With this change, she also wouldn't get such safe trades, because she'd either have to be up close to you already to get the QQQ > W > E away combo for safe trades or she would have to use her E to get in range of her Qs and then she wouldn't have the safe disengage for trades. You could still use minion waves to proc the following Qs but that would force her to naturally push the wave if she's going aggro kind of like how Yasuo has to push the lane if he wants to be mobile, and that would open her up to ganks more.
Just my 2 cents, but I honestly have always felt like this was the real issue with her. Its not that Riven is broken. Her Q combo just makes her incredibly frustrating to play against and doesn't have any counter play other than to be a mile away from her.
