Wouldn't making critical strike damage a base stat rather than a set value fix a bunch of problems?
By making it a base stat, and thus having it scale with champion level, you limit early power while letting it scale into what it needs to do. Lethality functions this way so that it's not as abusive in high values early game, which as I understand it is the current problem critical strike faces.
Basically, you give ranged champions +50% critical strike damage initially, and melees +75% (on live it's +100% by default, for comparison), and have it scale up to the normal +100% at level 18. That'd be roughly ~3% per level for ranged and 1.5% for melee, give or take a couple decimals.
Could even use different values than this baseline for certain champions, like having Jhin's critical damage scale higher than usual due to not utilizing AS in a normal fashion.
This also counts as a nerf to both Infinity Edge and Statikk Shiv, too.
Just a concept, spitballed numbers, it'd probably need tweaking.