Radical Idea: Completely overhaul the item/rune system

No. Whatever you're thinking, it isn't that. I mean completely.

See, one of the most enduring points of balance aggravation stems from the fact that these game systems are purely modular.

Anyone can pick up any item/rune, even if picking that choice is an absolute trap-option (haha, what an idiot, you got on Vayne). And, of course, there are examples where the converse is true. An option works too well with someone. Because of this, you get situations where Riot is left with one of two choices. Either you destroy the modular option, ruining it for everyone else, or you destroy the champion, making them completely dependent on the modular option (and then for good measure destroy the modular option, too).

This modular system has caused heartache and headache to no end. I'm pretty sure everyone is aware of Ezreal filter for adding any form of OnHit item to the game. And of course there was the whole fiasco with Kog'Maw's rework. Aaand then we've got the penchant for making Tank items terrible (while giving tanks more in-kit durability) to discourage non-tanks from picking up any defensive items ever which has helped contribute to our lovely 'everyone dies instantly' state of the game.

So.

Radical idea.

No more modular item/rune system. At all. Period. End of discussion.

Instead, each champion has a suite of items tailor made for them. Obviously a suite of items much smaller than the total pool we've got available right now. The items grant some stats and interact in some way with existing components of the champion's kit, adding new and interesting options. We saw something akin to this in one of the recent rotating game modes and it was received pretty favorably, I think. Because items would be tailor made to the champion in question balance could be addressed in a much finer sense. Particular combo too strong? Nerf that specific item.

Something like, say, three or four items keyed to each champion's abilities and passive. This would still allow for a tremendous amount of variability in champion build, and ideally this variability would actually consist of viable, useful options rather than wondering whether or not grabbing on Annie is a good plan (it isn't).

This would allow a tremendous boost to champion build diversity while giving much more finely focused balancing tools.

Downside?

1. That's a lot of work.
2. Leaving old, well understood systems behind is uncomfortable.
3. Seriously, holy shit, that would be a lot of work.