I don't get the whole mandatory secondary thing. If someone wants to play a specific role, just let them. 'bu bu bu queue times!' I'm sure a lot of people would rather wait a little bit longer than be put somewhere they're not as good at, and I'm sure the rest of your team agrees. "omg why is top doing so bad" "because i queued for mid". And sure enough, they're probably a mid placed top against someone who mains top.