Don't let players into ranked games without complete rune page(even if only tier-2 runes for 1 IP)
When Flex queue hit with it's fresh MMR, I noticed that some players enter ranked games without any runes. Now that tier-2 runes are essentially free, the only reason I can see is that such player doesn't know anything about rune system.
Giving them a reminder(and a requirement) to complete their rune page, even if only with tier-2 runes, would make game better. For them, mostly.
Alternatively, empty rune slots could be occupied by default with HP runes when entering ranked, or so. But I think that forcing them to fill it isn't too much of a burden, and actually is a learning experience.