The Senario: While exploring the bajorian wormhole the federation encounters the empire.
Rules: Canon material only, no temporal interferance.
Timeline: Hight of Empire, just before the battlend of Yavin 4.
Deep Space 9 just before the first encounter with the Dominion.
Additionally: The Q set it up so they will no get involved, expect to annoy significant characters. The rebellion still exists but they are severely weakened as the empire will retask the death star and send a fleet of star destroyers instead. The klingons will quickly join the federation, the Romulus will but it will take time. The Cardassians will ally with the empire.
Spock, who helped negotiate the use of the cloaking device for the Defiant contacts Picard.
They assemble a team, take the Defiant and infiltrate the Section 31 research station where the Planet Killer has been studied and it's computers partially restored.
Data and Spock manage retrieve the general location of a number of other Planet Killers from it's database.
The Enterprise and Defiant leads a small fleet of ships to find one reprogram it for an attack on the Empire.
They first go to Betazed where Wesley Crusher and a large portion of the population reach out to the Traveler. They are of course unable to locate him, but they are able to get his attention. It works and he joins them in a few days.
With his help they quickly travel to the border of the Gamma and Alpha quadrants on the outer rim.
There they locate a Planet Killer in a few days.
A group of Tholian ships managed to halt it's movements, while the Bynars use a Borg device to overwrite it's control systems.
A multi pronged attack on the Empire.
The Traveler's energy is taxed from the retrieval operation so he is unable to help in the attack.
Several cloaked Romulan ships carry teams of Betazoids to confuse, defend against and attack as they are able the Sith Lord and Darth Vador. These ships are scattered throughout the fleet.
Teams of Bynars are employed to deal with Droids and other Empire tech.
The Defiant and much of the Klingon fleet engage the smaller ships and clear a path for the Planet Killer to destroy the Death Star.
How's that?