His replacement would probably be Rand Paul. And given that the GOP is made up of many many Libertarians, he might win the nomination. I think this may be a mistake though because Libertarians (generally) are socially to the left, and this might keep conservative minded Christians home on election day...again.
If Paul ran on the Republician and Libertarian lines ( in fusion States/Commonwealths) he would do very well.
this could be a good time to have elected officals change laws not for fusion status.
On the Dems side:
Hillary has said she was tired of politics or words to that effect. But its interesting that she's retiring. She could find that a 4 year rest is enough and would be a difficult candidate to beat in '16.
She might use Reagan as an example - as he was 69 at inauguration
Cuomo will probably run as well. But i think the nomination is Hillary's if she wants it.
New Yorkers love him - but he divorced a Kennedy