Over the course of 7 seasons, Riot should have tons of data saved up as far as balance goes. This is probably wishful thinking, but what if they used said data to build a program to determine the effects of an update? Kind of like what the pbe is except they get to see the effect without actually having to implement it.
From a programming perspective, it would take a shit ton of work, but it's ultimately just a database that can run calculations. Let's say they wanted to nerf rengar. They could put the nerf into the system, and have the program run through what the most likely effect of the nerf would be on everything else in the game.
For instance, Rengar has his ult duration nerfed by 2 seconds. The system would output data to show how that would effect rengar's win rate and everyone elses. Rengar's winrate drops 2%. Champions that Rengar counters gain a 1% increase in winrate. The build path might switch to accomadate for more movement speed, etc. That's really dumbed down because changing one thing would effect a lot more, but with a program like this, Riot could avoid over tuning champions all together.
This would take a shit ton of time to perfect, and because they have the PBE, they could do test runs whenever they wanted to make sure the system is working as intended. Even if they did go to work on this, it probably wouldn't be for several seasons until the program is actually operational. The question is whether or not Riot wants to set aside the time and resources to build something like this, but if we're talking about long term game health, this would be one of the best available solutions. By no means would this be the end all way to balance a game. There's still going to have to be intensive work done in the same way it is now, but the system would be a good way to judge whether or not a change is necessary or not.
Feel free to offer any adjustment or anything. I might think of more things later so chat with me if you have questions.