Jensen Ackles would be a good fit, but in my mind, he's in the same boat as Michael Hall. I look at one, I see Dexter. I look at the other, I see Dean Winchester. I can't really see any way around it in my head.. It's been, what 10+ years since X-Files and I still see Fox Mulder when I see David Duchovny, despite the 3 or 4 of his movies that I really enjoy and watch a lot.
I REALLY think that Jack should be played by an unknown actor. It kind of fits with the theme of the character.
Abe, however, in my mind, has Ken Davitian written all over him. I've already cast him in my mind, he's who I see when I imagine the RJ scenes in my head.