This is a combination of "challenge your leader" and "choose to attack your game center
I think a good idea would be to have the ability to look up friends or just browse people in your skill level/range and send challenges. If they accept then you get notified and can attack them but they can also decline if they don't want to be attacked by you. Also to counter the idea of spamming someone until they let you attack them would be the ability to not accept challenges.
After looking at some of the responses to the other two I thought this would work because it combines them but to me seems a little more fair.
Plz give me your opinion on my idea.
in your inbox there should be another area for ppl sending challenges. If you decline a challenge it gives you the option to block the person. If you accept then it notifies them and they can attack you. (Battling will have to be worked out) (I'm thinking it might just be a way to battle specific people)

