Let's say Hillary won't be the 2016 Democratic nominee, who are the Democrats to watch for?
Andrew Cuomo (58) New York governor. He's from a famous family; knows how to play hardball, and would be tough for any Democrat in the primary or Republican in the general election.
Jerry Brown (77) Governor of California. He's worn that office 4 times! He'd be a consensus choice in an emergency, who wouldn't threaten anybody's political future.
Claire McCaskill (62) Missouri Senator. She has a well-won moderate reputation but acceptable to the left. She'd be a great vice-presidential choice this year, and clearly Presidential if somehow the party needed her.
Tim Kaine (58) Virginia Senator. He is a good vice-presidential pick this year. He would be a great candidate for President were he needed. Former governor, former DNC chairperson, and current Senator with a good repuation across the political spectrum.
Furthermore, in 2020 or 2024, who are the Democrats to watch for in any case?
Corey Booker (46) - New Jersey Senator. If he can win an impressive re-election in 2020, and keep building his moderate image, at age 55, he'd be prime to run for President in 2024. He'd also probably be on the vice-presidential short list in 2020.
Kamala Harris (51) - California Attorney General. She is running for US Senate in 2016. If she wins that race, she instantly becomes a national superstar. She'd definitely be on the vice-presidential short list in 2020; and if she won an impressive re-election in 2022, prime to run for President in 2024.
Amy Klobuchar (55) Minnesota Senator. She is ready now to burst onto the national scene for real! She should be on short list for vice-president right now, and a cabinet pick if a Democrat wins the 2016 White House race. If Democrats lose in 2016, she'd be ready for 2020, assuming she wins re-election in 2018.
Tim Kaine (58) see above. He needs to watch out for what happened to his colleague Mark Warner, who almost lost re-election to a political novice who outworked him. (Even though his opponent was a former GOP chairman, no one expected a razor close race)