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.
Only tangentially related, but on the subject of accents: I'm doing my best to hear Akaar's dialogue in my head with a Russian accent, since I picture him as being played by Johnathan Hyde, as he played Ilya Gavrik in the 10th season of MI-5/Spooks
, mainly because of Gavrik's relatively deep voice. (And he's pretty tall, too.)
I've been making efforts to hear Choudhury with an Indian accent, though it now occurs to me that her accent is identified as "Denevan," and who knows what that sounds like in the 24th century.
I read First Minister Asarem's voice in the voice and cadence of a friend of mine who is from Nigeria. As I recall, she is described as having rather dark skin, so I imagine she might be played by a Nollywood actress.
Generally speaking, I'm trying to give different characters distinctive accents and voices in my head, to make them stick out more in memory.