Children of Bodom sure as fuck don't sound like death metal to me...sounds like black metal to me...and I don't like em that much, so I highly doubt that memnoch would.
I haven't heard a female voice in a metal band that I've liked, and I've never heard Arch Enemy, and not planning on doing so anytime soon, so don't throw Angela Gassow at me.
Dream Theater uses a lot of keyboards, I should know, saw em live, but I'm not quite a fan of theirs either, unless two songs out of virtually 4 albums makes me a Dream Theaterhead
And 'noch, if most of what you listen to sounds like Immolation, you're right, it probably would be ruined with keys and/or females.
I like the way Dimmu Borgir uses keyboards, check out Mourning Palace and Indoctrination for anyone who hasn't heard them that's interested, and I like the way Morbid Angel used keys in Dreaming and a few other short instrumentals, but that's about all for me.