Wow, someone who actually liked DoC o__O I always thought I was the odd one out.

As for rank ideas, according to the FFwikia page, much of the organization is split between military members, researchers, and espionage, so you could always go with military-themed ranks, like officers and lieutenants. The bottom of the page has a WRO member listed as "Commander" and the other two simply "Member", so you might have to get a little creative depending on who's in your FC and what role they want to play, if everyone is equally serious as you about the ranks making sense.