I posted this idea several times over the years, but I always envisioned the Romulan faction being one where a Romulan player could infiltrate the Federation and spying as a Vulcan officer. Then reporting back to the Romulan High Command. And sometimes you benefitted as well.
For example, you stealing the secrets of Slipstream technology from the Federation and that's how you unlocked the ability.
That would be brilliant I was personally hoping that the Romulan faction would have a lot of infiltration missions where we sabotage or steal fro mthe KDF or FED, kinda like the mission where you have to use a holoprojector to infiltrate a Klingon base to sabotage the doomsday weapon in the FED storyline.
Make 1 fed faction. Have a kdf and romulan "sub faction".
Give players of sub factions a starfleet rep.
When you get to certain levels of the rep give them access to teaming and missions and fleets and earth space dock and starfleet academy.
Rinse repeat for all factions.
Since whats done is done id start from this point. The Klingons being the bad ass expansionist that they are and fearful D'tan may be leaning to closely to the feds institue a coupe attempt that fails. It does however expose a Section 31 operation attempting to influence and sway the Romulans to the federation as full members.
D'tan pulls back from his alliances and enters a ceasefire/detente with the Tal Shiar. The Klingons become openly hostile and the feds become more neutral as they rein in section 31 and decide how to proceed forward.
Eventually D'tan creates a new senate under pressure and the new romulans are a hodge podge of conflicting views, save one, to rebuild the republic in a way that preserves freedom of the romulan people but paves the way for uniting the empire, through diplomacy if possible, force if not. Only united can they stand and the "empire"/Tal Shiar will not sit idly by.
The new romulans then find themselves battling the Tal Shiar from within and the Klingons from without. A tal shiar plot threatens the federation and section 31 is redeemed as they discover the plot originated on new romulus and may possibly involve some of the new senators. Open hostilitiy ensues with the feds though no declaration of war exists.
Of course this means the romulans are an independent faction, but have limited resources, The in game romulans will grow as things are added by the dev team, star bases and such.
Simple... I would implement it as a full faction, level 1-50 with fleet holdings, independent doff system, independent explorations system and standing completely on its own rather than having to joint the KDF or UFP in their stupid war that has had no emphasis for 3 years and makes no sense to keep going except as an excuse for PvP...
Those of us who see things for what they are need to get it through our heads that no matter how much we rant about it, THEY WON'T CHANGE!. We can either live with it, or we can go find some other game to play until it too embraces the same tactics. The reality is, STO is what it is, and it will not be anything else, no matter how much some of us may want it to be...
Okay, keeping in mind that I've only progressed in the story as far as going to Mol'Rihan to help D'Tan out, and have no access to Tribble and thus no idea how they're building their story now...
As I see it, the collapse of the Romulan Star Empire is inevitable, from the moment ch'Rihan is destroyed. The Empire's too centralized to survive the loss of its capital, especially when a half-breed like Sela has assumed the title of "Empress". (Don't look at me like that - I'm projecting Romulan thought processes from available evidence, and it doesn't look like they'd be happy about being led by someone who's only half-Romulan.) So, as my ideal story opens, you're a young erein (lieutenant, roughly), commanding a minor ship on the fringes of what's left of the Empire, when you get word of the disappearance of Sela. Your first mission would then be to find and execute the Tal Shiar political officer on board your ship. You then can try to build a new Empire, using the resources available to you, taking you on the 1-10 course, or perhaps as high as 20. At that point, you're butting up against every other would-be Emperor in your sector - and some of them have allied against you. Just as your situation begins looking its bleakest, an envoy from D'Tan comes, offering you assistance, on the condition that you bring your ship to Mol'Rihan and agree to cooperate with his allies in Starfleet and the KDF. Agree, and you'll have access to all the lovely new tech and ship types his alliances offer; refuse, and you're on your own against your new enemies.
(That's called "Hobson's choice" - a "choice" that really isn't one.)
Wanting to survive, you take the envoy up on his offer; he gives you coordinates for Mol'Rihan, and when you get there representatives from Starfleet and KDF are there to recruit you to assist them. (Any missions against other Romulans at this point can be excused as being battles against fools misguided enough to follow the Tal Shiar's lead, and too stubborn to see the error of their ways and join you. You know, kind of like those missions currently in the Klingon storyline where you have to spend your time shooting at Klingon ships from the House of Torg.)
This implements a Romulan "faction" that fits into the storyline as established, giving Romulan players a unique starting point, while requiring only cosmetic changes to later missions (mostly having to do with dialog).
------------------------------------------- I'm old enough not to care too much about what you think of me --
But I'm young enough to remember the future, the way things ought to be... - Rush, "Cut To the Chase", Counterparts
I'd have it where there was a mechanic where by PvP and other various activities you could become "Praetor" or "Emperor" of the RSE for a short time and gain special benefits in doing so. However it's very hard to keep a hold of.
And if you don't feel like doing that you could always join D'tans space hippie group.
1. As a separate faction, with their own fleets and starbases.
2. Each individual player gets to choose which side to ally him/herself with, Federation or Klingons, which gives access to that faction's quests/content, but not their ships.
3. For PvP, the Romulan players get pooled together and make their own team, and can end up either facing the Feds or Klingons. When not enough players are queuing for PvP, Romulan players can be grouped with either Feds or Klingons. (Depending on which side they are allied with.)