Basically, who do you think deserves their own Codex and why? Some Chapters don't need their own Codex at all, because their organization is Codex-compliant and they don't have special things the way some other Chapters do. Some Chapters, however, have different organization and make good candidates for their own Codices.
Black Templars, for example. Now, I don't play Black Templars, but they definitely deserve a Codex the most out of ANY current "Codex: Ultramarines" chapter. COMPLETELY different organization and doctrines of warfare, combined with an intensely cool aesthetic and lore, as well as overall popularity. They have special units all their own, and they have their own version of a Captain entirely separate from Codex Chapters, so... I feel they should get their own Codex.