I also really like the lines of this ship: Defiant, Miranda and Jem'Hadar put together, as others have mentioned. I can see the back story being that the designers were able to incorporate technology from captured Dominion vessels.
I like was JES said about the naming and numbering. I got into a discussion about reusing names in another thread, so I agree that you should avoid the earlier name unless it was an especially significant ship.
I like the roll bar, it really gives a nice line to the ship. That darker squarish detail in you latest picture looks a little Star Destroyer, though. Does it have a particular function you are representing?