I would really like to be able to play X (3?) rated games per day against my friends (maybe x per day per friend?).
The reasoning behind it is that I really like the friend list, and I made tons of friends already. But since I only play ranked games, I rarely ever meet them. Thus, if I could challenge them for some limited games per day, the friend list would make a lot more sense for me.
Now I understand that this still leaves a possibility for abuse. But abuse will always be possible, and it seems that with that system, abuse related to playing against "fake friend accounts" would have rather high effort for rather low gain.
I get that this is not going to be on your high priority list, but maybe we could discuss and refine the idea, and when the time comes, it might be interesting to have a look at it.