KDF needs more fleshing out to do before the Romulans can come. I and hope DStahl doesn't entertain his idea of a "mini faction".
I think a mini-faction is the best we can hope for. The amount of work to make the game compatible for 3+ factions would be huge. And as other have pointed out the KDF is missing a lot of story content. So expecting another current KDF level faction now is naive, in my humble opinion. Cryptic just doesn't have the staffing to crank out that amount of content in addition to supporting the current areas of the game.
A mini-faction with content up to level 11-15, and then make a choice joining a faction which is aliied with the KDF or Federation, I can see. In terms of the state of the Galaxy in the game, the Romulans are far to week to be able to sustain themselves as an independent galactic power. Especially now since the Tal'siar has been a rogue group a lackies for the big bad under Hakeev. The Remans are quickly establishing themselves out from under the thumbs of the Romulans. With Sela missing all of the infighting between the power bloks will resume and distant Romulan Colonies have been and will continue to want to be independent. Not to mention that Jm.pok may want to start making grabs of Romulan territories along their mutual border. And there is a wildcard of the assimilated Donatra that's escaped from the destruction of her Scimitar.