Maybe Dinah Madani will be like a revamped version of O'Brien?
I see no reason the show can't have Castle having some type of romance. Castle might be a rage-filled and driven dude that's become a vigilante monster on his war on crime, but he's still a dude. He still has a soft spot for de ladies. He still has urges and needs to let go sometimes.
It will probably be like Spartacus where he never truly loves anyone as much as he did his wife, but he can still get involved with some other female characters.
Just like I never understood Batman can't romance or have girlfriends, why can't Punisher? It was one thing that annoyed me about Hush. I was actually getting sort of excited about Batman and Catwoman because Loeb seemed to write them well together, then it all falls apart at the end.