For me personally, it'd be limited to players with 5* gear. Rank advancement would be based on achievements such as soloing certain levels or skillfully leading a party through a Shadow Lair and other stuff like that. And since I'm too lazy to be a Guild Master, there would a be a leader who administers the rank tests, enforces rules and good codes of conduct to safeguard the guild's reputation, and why not, randomly gives away prizes to online members. And since I rock a Heavenly Iron Helm and Radiant Silvermail as my costume, it'd have some cool name like "Paradise Knights" or "Divinity Defenders".
And last of all, let me thank and praise Ashiram for being just a such a player to create and manage such a guild. Thank you, Ashiram, for "Cloud Nine".
Perfect guild for me.... depressed when I am just logging on, quite happy after having a guild chat. ;p