A possible end for season 5...
Kate is pregant!!!
It would be epiicccc!
No, it wouldnt it would put a lot of pressure on them and ruin their relationship. I expect Castle proposes in the middle of Season 6 (next Valentines Day, perhaps?). Then they get married at the end of Season 6 and she gets pregnant at the end of Season 7!
No Caskett baby, not yet. I agree with Alex.
People always want the babies, I guess to assure that their OTP stays together, but I think it would be much more meaningful if they dated for a while and then move in together, and then maybe get a dog, and then get engaged, then married, then Caskett babies. I'd feel like they were forced to be together if they brought a baby in too soon.