I think I've figured out how they will eventually end the series. They will wrap up the Kate's Mom storyline by having Castle's Dad shoot the Senator, and for some reason it will be a sanctioned hit.
And they all lived happily ever after.
Don't think so.. given that Kate didn't shoot him right when she had the chance she will find a way to publicly expose him and his doings effectively ruining him forever. Maybe he'll overreact then and she still gets to shoot him but then it'll be self defense and not murder.
Oh and the finale will include Castle's and Beckett's wedding and/or announcement that she's pregnant.