How about giving players an option to craft a mod with the values they desire for a bunch of credits, materials and maybe a store item for a few plat?

Just make the results a tad weaker than the optimal Riven mod and make it so you can either equip a crafted or a Riven mod. That way Rivens stay almost as attractive as they are now with the (expensive) alternative to create an (untradable, less effective) customized alternative.

Community content is available under CC-BY-SA unless otherwise noted.