In regards to unmasking the Breen and removing the mystery, I think back to the words of Spock: "After a time, you may find that having is not so pleasing a thing after all as wanting."
It could go either way. Someone around here postulated they were proto changlings. That's pretty interesting. There's other cool ways to spin it, and lame ways also of course. Like all this stuff, I think "Ok, now what?". I hope what they come up with outweighs the fun mystery of the Breen.
One thing that always bugged me about the mystery of the Breen is that it shouldn't actually be a mystery - Kira and Dukat take the refridgeration suits off a couple of Breen in "Indiscretion."
Heh... Yea, I forgot about that. Trek isn't always the best at consistency, or perfect logic. They could still pretend that never happened, or give some hand wavy explanation for why Kira and Dukat never mentioned what they look like.
It's out there now. I guess we'll see where it goes. I'm not upset. There's been worse design decisions in the franchise :^)