tl;dr Mastery based chest system, (over S based)
I love the fact that Riot gives us the chance to get chests and loot for free, albeit at limited rate. but for the entire time I've played league, a couple things have bothered me in how chests are earned. To the best of my knowledge, the chest system is supposed to promote people playing a variety of champions, and getting good enough on said champions to get an S rank.
I believe that this system is not very good in its current iteration. I will propose a solution to improve the system using only
Currently, you get chests by playing a game on a champ where your you or a friend gets an S. Additionally, you can only get 1 per week (storing up to 4)
I believe there's a problem lies in the criteria that chests are awarded on. S ranks are supposed to mean... you're good at the champion? maybe? I'm pretty sure it's based on CS, kills and damage. Every champion also has their own requirements for an S rank, which are hidden from the players.
In my opinion, due to the snowball oriented state of the game, and the lack of definitive stats, S ranks mean very little, and your friend earning an S rank means even less than that.
I'll play ARAM with some friends and if we stomp them hard enough, someone is bound to get an S after a few games. Especially if someone on the enemy team rage-quits, or disconnects, or whatever.
Instead, I'd like to propose a different system
You get chests after getting a certain amount of mastery points on a champion each year.
Personally 50,000 is my reasonable number (about 50 wins). However if this is too low, or too high, Riot can change it to any number they want
(for each champion, in your collection, a mini mastery bar might be shown with a % when you hover over it)
This rewards players that become good at the champion because they gain more mastery points for wins
This encourages someone to play a variety of champions, because it's still one chest per champ
This makes the requirements for getting a chest crystal clear.
This might increase the diversity of champions in normal games because 50,000 is kinda hard to get solely in ARAM,
If letting people get too many chests is a huge problem, (through botting), the weekly chest limit can be added back. In this case, once the mastery bar is full, it would highlight green, and the requirement would be "play one game with this champion with a chest slot open"