Here's a few suggestions.
1. Most importantly, a reliable system, where once you build the database (Or However you are doing it) the tags will work within a reasonable amount of time, on all servers.
2. Allies (If wearing a tag, you can't hurt each other, maybe the idea of allies could be expanded eventually)
3. Complete the Private Message option, that hasn't been completed in the current GuildCP.
4. The Leader can set any member's nickname.
5. GuildCP isn't in a your browser. A downloadable version would be great to have.