Tutorial Suggestions
One aspect this game has always been lacking in is its tutorial. Rather than the actual basics, it covers only the barest of bare bones. But a tutorial that covers only the fact different champions are different and that the objective is to use items to power up and destroy the enemy's structures isn't nearly enough to inform new players of how the game actually works.
My suggestion is that there be somewhere around ten short tutorials that cover the aspects of the game which remain largely the same even when champions, items, runes, or the jungle are changed. Each one would be short enough to explain the point without feeling like a drag to get through. And the Tutorial listings page in the client would have in its description something along the lines of, "these tutorials are not meant to be required reading, but if you find yourself struggling on a particular topic, check them out." The aspects I can think of right now that would be covered would be (in no particular order):
Counter-building. The player is put up against a Yasuo bot with IE and PD as a fighter who doesn't have Randuin's in their Recommended page, and walked through how to search for it to counter an enemy with high crit chance. And then against a Karthus bot with Adaptive Helm being the item. The champion, level, and other items of the player could be locked in a state (changing between the enemies) that would make fighting either champion doable but difficult without the defense item.
Minion control. The player would be placed into three situations: pushed to their own turret, held in the middle, and overextended themselves, and shown how to deal with them accordingly.
Crowd Control. The player would be Xerath put on a team with a Leona bot against a retreating enemy, where it's explained to the player that they should attack enemies who have been rendered immobile by allies, and use their own crowd control in a way that doesn't waste it. The Leona bot would then E-W-R the enemy, with the player instructed attack while they're stunned and use their own E when Leona's stuns end.
Balance Patches. This wouldn't be interactive, but a short video explaining with one particular example how important it is to stay up to date with patches. The player will be shown how balance changes can completely change builds, with the best example being the change that turned Morellonomicon into a specific counter item and replaced it with three other first-buy options.
I know there are more things to add to this list, so please mention those I missed in the comments.
On a side note, I want to point out how this is apparently "a suggestion, but not a discussion topic" for Gameplay+, but topics greenlighted for + include a flat description of an item that poses no questions or lends to any kind of discussion, a thread about crit that is no less a suggestion than this, a complaint about Zyra's plants, a suggestion for a Tristana rework, and many other suggestions that aren't any more discussion-worthy than this. I'm not really sure why my account in particular is being treated unfairly in regards to gameplay+, but oh well. Evidently "suggestions" aren't to be treated as discussion topics anyway.