I'm going to be blunt. This idea sucks.
Riven does not need attack speed. Gameplay wise, attack speed is one of the weakest stats Riven could build. Also, riven's passive is her only real strength. Take that away, and she suddenly requires tank stats which ruins her uniqueness. This change would make every single one of Riven's bad matchups much worse and any of her survivable matchups where she needs to do short trades only unwinnable. Hell, she wouldn't even be able to scare off a Nasus at the start. Why should every single champ in the game have the ability to be a threat right from the start but not a skirmisher whose effectiveness is already based heavily on her player's skill level??
Why not instead give her a fighting game inspired super meter? Make it so this meter is filled like it is in a fighting game, whereas it rises by deal damage, taking damage and passively and slowly gain meter while under 35% pf her health? From here, she has her normal abilities as usual, but she can cast "EX" versions of those moves, which are simply stronger versions of those moves with either better damage or some other special quality over their weakened counterparts, at the cost of 25% of her meter and have the ability to cast either part of her ult at 50% meter? This way you don't kill her ability to trade while at low meter, but you also cap how much power she can bring to any one fight even while at full meter. Its a fun resource mechanic that is both fun for the Riven and a good way to allow her opponent a clear idea what Riven is capable of with certain amounts of meter.