In my dreams...
At first, Rick's dad will stop Beckette from shooting the Senator since he doesn't know the Senator is evil. The Senator escapes, Beckette explains what's going on. Maybe they'll work together to find find additional evidence that proves beyond shadow of a doubt that the Senator is corrupt. In the final showdown, Rick's Dad will stop the Senator's bodyguards freeing Castle and Beckette to take down the Senator. Beckette will be taken by surprise by the Senator and he points a gun at her head. He is about to shoot Beckette when Castle bursts through and shoots the Senator in the chest.
Yup, it will end with their wedding.