Riven slots really are a bad idea. If we get more mod slots on weapons, they should either only take utility mods or just not accept straight-up damage mods.
You want to increase build diversity, not make only dmg builds even more effective. Oooooh, now you can put Serration, Heavy Caliber, Argon Scope, Point Strike, Vital Sense, Critical Delay, Bladed Rounds, Catalyzer Link, your Riven and a bunch of elemental/status mods on there! Now every weapon feels EVEN MORE the same~! Yaaaay!