Please Do Not Make 5 Ranked Roles
This is a personal request and only stems from my opinion of the game and how it's played. When I queue into ranked, I know that I'm queuing into a game where I'll have 4 players on my team and 5 players on the enemy team that all have the same understanding: This is mid-diamond gameplay. There's an understanding that whatever role you get, you're going to have to perform at that level. If I get queued as ADC against someone who mains the role, I'm going to likely do worse than that person. I understand that. That drives me to learn and to improve throughout that same game and gives me crazy amounts of insight as to how to be a better ADC player.
I do not want to get queued against another ADC player who doesn't know what they're doing. I also don't want to get autofilled into a game knowing that I'm not going to be ranking up the rank that I want to, and I have to sit through the next 30-40 minutes of playing a role I don't enjoy until I can get back to doing what I was doing.
Of course, you've already announced it, which means there's no changing your minds, Riot. I can only hope you find another way to make this system not as terrible as it sounds between now and 2019.