To me it was always clear they would keep Molly Q./Alexis on the show... she's an integral part of Castle's life and quite popular. I just hope they'd use Martha and her more.. they are a hoot and their scenes are always so funny/emotional/moving/etc.
I don't think that Castle is hiding anything anymore.. he's said his piece fully in the season 4 finale when he laid everything in front of Beckett. Maybe only the details are missing what he's learned but now that the cat is out of the bag he can fill her in on the details.. there just was not enough time in the finale to do so and then stuff.. happened
Given what we already know about the season opener they will talk some more and open up to each other and i so look forward to these developments.. i've always liked the writing on the show and think that their relationship is one of the more realistic ones on TV and within TV constraints so i can assume it will remain so and we will get endless hours of fun and drama.
About 3 weeks to go!