Terak Nor:
After the Dominion War members of multiple species come to DS9 to study the wormhole.  Their joint effort has lead to the creation of the Herald, a ship built with Federation, Klingon, Romulan, Cardassian, and Ferengi technology which carries a prototype wormhole drive.  When they activate the drive, however, instead of taking them to another star system in a few seconds it took them back in time 20 years.  They find out that a Cardassian scientist that worked on the Wormhole Drive project was an ex member of the Obsidian order who knew that the ship would go back in time, and has decided to use the opportunity to save Cardassia by assassinating key people within the Cardassian, Federation, and Bajoran territories.  We must stop them.

or

Delta Quadrant:
Several years after Voyager returns home Star Fleet develops Transwarp conduit technology thanks to the help of Seven and Echeb.  A ship returns to the Delta Quadrant to reestablish contact with the people Voyager met and attempt to use new terraforming techniques to save Ocampa.  Things aren't as they had hoped, though, as the destruction the Borg underwent, as well as their other interactions, threw off the balance of power in the quadrant.