I know I'm not the first to think it would be great to attack our friends. It could bring something pretty interesting to this awesome game. I think there could be a way to battle our friends without exchanging any resources because it would affect all the game and it would not be a good idea. I've come up with an idea that could do the job.
Firstly: To battle our friends, we could form troops and create spells without spending any elixir. We could call them "ghosts" troops. They could not be used in any multiplayer and clan war battles. You could only use them by attacking your clan mates or your friends.
Secondly: The ghosts troops and spells would be stored in the army camps and the factory in an other "setting" than the original ones. Besides, you could store the same number of ghosts troops and spells than the original ones.
Thirdly: The attacks would not affect the resources of anyone. You would not gain loot by winning an attack or lose gold or elixir by being defeated. With this "rule" no one could exploit the game by creating a new account and by exchanging loot with multiple accounts. It would just be fun to be able to attack our friends.
With the clan wars, we can fight with our friends but with this new feature, we could also fight against them.
Also, their could be a leaderboard of you and your friends so you could see who's the best.
At the bottom line, this new feature would bring more fun to the game and could help the players to build their strategies to make their way to the top.
