Romulus and Remus are gone. You are one of the survivors, struggling to survive in the aftermath of unspeakable destruction.
These are dark and dangerous times for your people, as the Tal Shiar crack down on any dissent and mysterious beings unleash terror throughout the far-flung colony worlds.
You will be the one to reshape an empire. Gather allies, go undercover and find the proof that will rally your people to revolt.
Together, you will rise up against an oppressive regime and demand freedom. You will fight for your people.
You are the Legacy of Romulus.
Romulan Republic players will be able to take on either Romulan or Reman characters. No news yet whether the new faction will absorb any other species - The current Klingon faction for instance includes Orions, Gorn, and Nausicaan, among others.
There will of course be lots of new Romulan ships, already previewed are the Enterprise and TOS birds of prey, and the D'deridex and Valdore class warbirds. There are also some new designs in the trailer, as well as, I think, the Romulan shuttle seen in DS9 once. There aren't that may more canon designs to pull from (D7, science/scout ship, Scimitar, Scorpion, drone, Narada), so I'd imagine most of what's available will appear too. The Romulan ships will apparently have some capabilities unique to their design with the promise to "unleash the new and devastating powers of the Singularity Core".
There will be new Romulan focuses stories, including the return of Denise Crosby, this time voicing Empress Sela.
There's also to be a new adversary, which are dubbed The Silent Enemy in their section of the Romulan preview site:
Uncover the horrifying truth behind a mysterious new enemy that seeks to terrorize the Beta Quadrant and exterminate your people.
Between that title, and the new ship designs I can't help but hope this is a follow-up to the mysterious aliens seen in Enterprise's Silent Enemy.
Though that may be very wishful thinking, as there's also this concept art for alien drones in the same sort of colour pallet as the new ships:
There will also be a new reputation mission featuring the Tholians
It's not all about Romulans, the update will also mean pleayers can start from level one as a Klingon as well (previously you had to play Federation until 25), there's also an update to the user interface.
Finally, for some reason also featured on the preview site for this expansion is a bit of concept art for a new insect...