... where there is only one class ... Inquisitor.
The profession is perfect for an RPG. You will travel all over the universe as your duties require, dealing with the threats to the Imperium in whatever ways you see fit. You can recruit NPC acolytes. Occasionally work with other Inquisitors. Even call upon the Space Marine Chapters in special circumstances - this probably has to be scripted; it would be kind of nuts to have that level of power all the time.
Why hasn't anyone done one yet?!
Edit: The reason I suggested only one class, the Inquisitor, is because it's the one job that involves much traveling, recruiting of party members and has few to no constraints in regards to behavior. The perfect fit for an RPG.
In contrast, "jobs" like the Space Marines aren't going to work in an RPG, the only "exciting" part of their job is when they are called to the battlefield (and they are always in a unit; so no soloing), otherwise they do what real life marines do, train - which is about as interesting as watching paint dry from a gaming perspective.