I love this idea! It would finally give meaning to the name of the game "Clash of CLANS". I also like the ideas mentioned by others in this thread such as requesting certain troops, and being able to chat with other clans. That way you could have a main clan, and then a feeder clan that tests out players before they are able to join the main clan.
Edit: Actually, I think that there shouldn't be the option of "Friending" clans, because then there would be the possibility of top clans friending and then only being able to attack other clans.
Also, there needs to be the option in the Attack menu of attacking "Enemy clans" or just regular attacking, because if you can only attack enemy clans, that wouldn't be fun. There could also be a limit of how many times you or your clan can attack another clan within a day/week. Let's say each player can only attack the enemy clan once per day.
Another problem that occurs is that you could visit the enemy clan member's bases, study them, and then easily raid them. Even if it is made that once a clan is an enemy clan and you can't visit their bases, you could just back out of your clan, study their bases, and then join back into the clan.
These problems would need to be worked around in order for this to work.

