Okay, here's a casting subject that hasn't been discussed: what sort of voice/accent do you expect from a Breen? (One that speaks basic, not that electronic noise from DS9).
I tend to imagine some with a Russian accent.
Honestly, I tend to imagine a standard Midwestern American accent. I see Breen society as an exaggeration of the modern American concept of meritocracy. As with the Cardassians, I don't imagine them to have a non-American accent -- though I don't tend to imagine the Breen as having the somewhat British-influenced style of vocabulary that the Cardassians often have.