My stance as a Riven main with 370K points and counting:
Riven's biggest weakness right now are heavy tanks in lane due to her need to snowball, and because they build a lot of armor/HP early on her ult does very little damage to them even when they fall below the max threshold (25%).
This won't do much except in the case of heavy tanks, due to them having more HP at 25% than any other champions. Regarding bruisers and squishes this won't do much because their 25% HP is much lower than a full tank and if they are at that 25% Riven's ult will kill you regardless. I doubt this will change her winrate more than +1% / +1.5%.
However a lot of Riven mains like myself don't want this buff to go through, at least not as is, because we're afraid it will push her into the "must ban" spot simply because people don't want to play against her.