Sorry to mention this first, but the no new members and AI problems are very pressing for your community and game, and dwarf anything you can improve here. Please fix them so that things like future alliance improvements actually matter.
However, I really do like the new alliance features, they were sorely lacking for leaders! Last online time, and alliance mail are very helpful. Here are two suggestions that are hopefully easy to implement:
1) Please allow us to do more than just an alliance tweet. I think I can fit 1-2 sentences in the mail. Its not really enough by a far stretch, needs to be 4-5x the amount of text to formulate an attack plan, advice on bases, etc....things that would be ordinarily sent on mail.
2) Because of #1, I often send out 2-3 mails to formulate a thought or message to the alliance. But, there is a max of 3 messages. Maybe either increase the max messages, increase the duration they are available (its 24 hours now I think). Id just like some sort of way to better leverage the system to communicate.