It's all for fun, going to be honest.
Kalista is not.. over powered? You can see the high skill cap on playing her to a degree that is above average.
Although one thing that's NOT fun at all is when you fight someone whose good at kalista - but that's rare in low elo due to the hole "she requires skill."
Yasuo's are more common, but I think a simple fix is just removing his shield completely and making him a mana less/rage less champion like katarina.
That shield is seriously bullshit. Specially when yas is on 200 hp - cait is ulting him, and he gets the shield up in time to save his butt and walk away with 10-20 hp.
Thresh is just.. lets not talk about op? because We've ALL seen a thresh who can land god like hooks getting smashed to bits.
Mordekaiser... op? uh.. sorry.. no. takes a bit of getting there - If he goes bot just take cait versus him and wreck his butt until he's crying and unable to leave the early game due to no farm.
Then give caitlyn a nami or morgana support... And well.. Sorry morde.
Darius - Darius is op.
Nuff said.