i have suspected long before any details came forward about this FE that it would be a Cardassian BoFF (not gonna happen as the story is entirely based on the missing dominion fleet), or a Jem'Hadar BoFF, all things considered, the Ketracel White object that only has negative values to non jem'hadar is a dead give away, otherwise whats the point without a Jem'Hadar on the side? other tid bits have come up recently hinting at the idea being stronger during the storyline (jem'hadar set, polaron rifle and armour, missing shield) and dev blogs, little words that others would miss that i dont ignore, there are similarities to the Reman arc for example like the breen arc.
but my prediction is:
last set piece
A special weapon or part (like the reman shield, transphasic torpedo, breen cryo grenade...)
i somewhat doubt a DoFF pack since i have never if hardly ever seen any mission rewards to any mission i have done include a doff reward pack thus far, why start now? (it also makes little sense considering its a c-store thing. what happens if its rerun enough to fill the entire ship up with dominion DoFFs? it would completely invalidate the c-store and i doubt cryptic are willing to let a valuable money avenue go to waste).