How about revisiting a species that deserves to be more than just a plot device?
The Hur'Q enslaved the entire Klingon race for approximately 3 years starting around the Earth year 1370. They eventually left Qo'nos, but the Klingons reverse-engineered their Energy Weapons & Warp Drive technologies, thus giving the Klingons a jump forward that probably would've took them another 100 years to do on their own, had the Hur'Q not invaded. This is probably part of the reason for the Vulcans' strict policy of the Prime Directive.
Anyway, the Hur'Q finally return from the dimension they've been holed away in, after 1,000 of genetic engineering to make their species even more formidable, only to find that their most prized possession, The Sword of Kahless, is missing from it's altar in the Gamma Quadrant. They immidiately run into the Dominion, overtake them, find information from them about the theft of the sword by Worf, a Klingon AND Federation person at the time of the incident, and proceed to the Beta Quadrant to exact their revenge on the Klingons, and reclaim their prize of Klingon servitude, and the Sword of Kahless, which is actually lost anyway.
In the first mission, you find out that the Hur'Q have also discovered the Borg in their travels. Since they are unable to use the wormhole by DS9 to cut short their trip, they pass through the Delta Quadrant in search of more "servants", in preparation for their inevitable encounter. In the first mission you also discover that the Hur'Q have actually discovered a way to control a small number of Borg at a time as their OWN mindless slaves.
The Hur'Q enslaved the Klingons for nearly 3 years.
They left to another dimension with the Sword of Kahless.
A Klingon Federation Citizen stole the sword back, and lost it.
The Hur'Q have returned even stronger to claim their revenge.
The Hur'Q control the Dominion.
The Hur'Q control SOME Borg.
The Hur'Q are coming to destroy the KDF, the UFP, and anyone else that stand in their way.
The only hope is for everyone to ban together against them, or die alone at their feet.
Does THAT sound interesting?
(My series of Foundry missions are based on the novel I'm writing.
Each mission is a side-story (Hence the name "Legacy Gaiden) which is based on the backstory behind the resurgence of the Hur'Q.)