I was thinking, just a picture of some kind, showing that your nation is in fact in an alliance. Not to show what alliance, but to show that you are in fact in one.
In my mind, I was envisioning having 4 or 5 basic member pictures, with CN stuff on them. They would be similar to the images that sanctioned alliances have, but would be generic, and just state something like: Alliance Member, Proud Alliance Member, or something along those lines. Nations that were not members of alliances could have one that states something like "No Alliance", "Searching for Alliance", "Accepting Invitations", or something like that.
You could even hold a contest to see who can make the best ones or something.
The only downside I saw with them was that they may take away from the sanctioned alliance status, as they will be the only ones that carry them.