If it's going to be a Boff, I'd like it to be an engineer to round out the trio with the Breen and Reman.
However, I'd rather have some other kind of toy.
Somebody actually was tossing this idea around in my fleet the other day: An orb from the wormhole that causes a temporal distortion (basically a PBAoE hold, like the Devidian cane minus the health drain, but also doesn't lock the user in place).
Not totally out of the question, I suppose, but I don't think it's likely.
DO's aren't supposed to be "game-changing" to the point where you have to play the DOFF system in order to play the rest of the game. That would make it very unlikely that a special FE reward would focus on one.
Now, as a regular or bonus reward for certain missions, like how we got the horta, that's different.
We don't know that the next FE's will have anything to do with Cardassians or Bajorans, only that DS9 is involved somehow. Something to do with the wormhole and/or the Gamma Quadrant seems a strong possibility as well.
I don't want to venture a guess until we know the theme of the next FE series.