"Premades blah blah, 4 chests per month, 1 chest per season" - let the trolling begin!
"Sorry guys, I know I main Evelynn, but I have to jungle Twisted Fate. I already got my Evelynn chest this season. Why play ranked then? I only play ranked. You'll have to carry me :(."
Also how does this even work with premades? If I get a "premade chest" because someone on my team used Evelynn, does this mean that I myself cannot play Evelynn to get a chest? And is there a way to even check and see which champs I already got chests for?
The premade thing is also going to cause MORE premades in ranked (as if it wasn't already an issue) and people will probably be more willing to queue with people an entire tier lower than them because "more chance for muh chest" making 4-man premade cancer more prevalent, and MMR gaps larger.
EDIT:
FYI: "You get a chest based off who you were playing. If I am duoing with you and I am jungle kindred and you are nami support and I get an S then your chest counts as if you got an S on nami."
This makes trolling more likely, since I will queue with someone who is really good on X and hope they get an S, and I will go "first time Y" to get my chest for Y