Would Masteries be easier to balance if you only got one?

As in, you get one Mastery point.

And that's it.

With that point, you get to pick one Masteries to serve as a secondary passive. Have something like, say, eight masteries in each of the major categories (DPS, burst, sustain, durability, mobility, utility). Or, y'know, however many it takes to fill things out. Since you're only dealing with one mastery, you don't have to worry about mastery interactions at all. And since you only get one option Riot could make the opportunity cost of each that much higher.

Even better, because there are so many options it would be MUCH less likely for champions to fall through the cracks where none of the Keystones really fit them at all. Especially with the Utility and Mobility options, there should be several choices that any champ would be happy about.