Make players play their roles.
May I suggest deciding what champs are meant to play what roles? Then create a system, that allows you to pick champs based ONLY on the role you've been given. That way you don't end up with an Eve support, Teemo ADC, Leona top, or some other horrible comp or decision. Although, if the combination of role and champ is unorthodox, then allow a vote system amongst your other teammates to either allow or not allow you to choose the mismatched champ. Make this system exclusively for Ranked. Because agreeably sometimes a game is just a game, but ranked is more competitive and should be taken more seriously than someone bringing down their entire team just because they don't care.
I've been so close to getting S4, but I've had A LOT of games this week where we get wrecked because players are just choosing obviously poor decisions without listening or caring. This system would allow better team comps and hopefully enforce careless players to play outside of ranked, where it wouldn't be enforced.