I'm the guild leader of a small guild (under 15 people). I'm also the leader of an alliance. In this alliance are four guilds, including my own. Two of said guilds are European based, two are based in the USA.
Now, I sign on at odd times of the day and night. Because my guild is so small (and generally not very active) it happens a lot that no more than 2-3 people are on at a time. Part of the reason why we made this alliance is to attempt to remedy that problem. We (the four guild leaders) all thought it was a good idea to join together for faction purposes and for adventuring purposes. (If my guild only has X amount of people and we need a few more then we can call upon the other guilds in the alliance to join us. Or we can just ask them in the first place.)
I've come to notice though, that whenever I start talking in the alliance chat rarely does anyone respond (with the exception of my own guild. Smartasses.

(And finally, the suggestion)
My suggestion is that the members of alliances be able to view the fellow alliance members that are on. Make it so that we can open another window with similar looks to our own guild window, but have it list the selected guild's members (on and offline).
I realize that this could lead to some abuse and harassment, but there are many things in Guild Wars that can lead to such activity so I don't really view that as a reasonable disagreement.
I also realize that some might say that I'm to blame. My erratic online times are certainly a problem. Not many people sign on at 2am (or 8am for those in my alliance in England). And while this is true, it also doesn't really take away from the idea.
If this is at all incoherent (I can't really tell) it's because I have a fever and it's 2am. Please, comments, suggestions, and criticisms (constructive, hopefully).

Thanks!