Yep. We do need some good Dark/Waters. Though I personally think we need more good Darks period.
It's pretty much my favorite type, and not many of the pokemon are very good. And yea, Shadow Jet sounds cool. (I always like Shadow-based attacks.)
Psychic/electric, huh? Yea, that would be pretty cool.
But, I don't think an evo would change from a Ghost/Dark to a Psychic/Dark.