You allready have a chat room for 50 allies, just keep them at 50 and randomly add people that join worldwide chat. Will keep the world chat controlled and shouldn't be to hard to implement I would think.
This will also solve the issues with alliances missing the 15/25 wars etc...
It will also help new alliances to be build faster and give them a better chance in succeeding. It's a enormous challenge to build a alliance these days.
Next release?
