This is amazing, and nicely balanced!
Hate it. Keep it 100% RNG.
I don't think Exotics should ever be left up to rng. They should all have questlines, cutscenes, and stories surrounding them; you'd acquire and work for them. Three of Coins should increase the rate at which legendary, exotic shards, and strike specific loot drop. It could also increase the chance and amount of motes and strange coins you'd get. Xur could sell trinkets that sorta temporarily boost gameplay, aesthetic appeal and giving good deals on ammo types. I still say he should sell ammo type consumables to put on your weapons instead of us even having elements on our weapons. Because what do the elements really do? They just take down a shield of their respective color faster, which means there's no need for elements on weapons. They could always adjust the shield and health of pve enemies to compensate. Now, if the elements had special, secondary effects, that'd be something, but that's not really the case here.