Just a quick idea from a noob, what about instead of a third faction they introduce the option of a civilian character, you would exsist in the world but you wouldn't be a starfleet officer, instead of rank the object would be money and you could work for whoever was willing to pay ( I guess you could call yourself a freelance captain, or mercenary). Your missions (or jobs as they would be called) could also be shady eg. smuggling, spy, raids, etc.. and when you eventually become a big enough pain in somebodies arse a faction could put a price on your head, and hunting you down would become a PvP mission.
Yeah, I think less levels. I think if you start Romulans, you should go ahead and start out at Captain level. This way they do not have to worry about adding tons of content for low level. You could do something similar to how the Klingons were unlocked by reaching a certain level in Federation career. Let's say you make Commander in Federation and you unlock the Romulan Faction and you start in Romulan at Captain.
Yes, there should be content to level from Captain on up but that is significantly less work in developing. Also, if they have the Foundary in place it is even less work.