Many people might argue that this idea would only cause chaos in every kingdom, but I believe that making it possible for a chief to request an alliance with another clan (the way you would request to join a clan) would help a whole lot; Especially with sending eachother resources - And perhaps even making it possible to allow allied clans to swap unused items with eachother.

I'm eager for this because my kingdom is suffering from the tyranny of a pay-to-win clan and all of the strongest clans (including mine) are joining together to solve our infestation problem.

But it's hard to get things done efficiently with the limits of the game's chat functions and the inability to share resources (maybe you could include an alliance chat with your own terms of use). If there needs to be a limit for balancing then perhaps just make it so that 3 or 5 clans is the limit to an alliance - and have one clan at the head.