HW was also the only one-term president in the bunch, though Jeb is having trouble getting his footing and his approval rating is teetering around 50%. Americans don't just elect Bushes and Clintons - they re-elect them. Bill in '92 and '96, W in 2000 and '04, and Hillary in '08 and a narrow win in '12 over Romney, who almost pulled the upset.
The easy way to cut the Bush dynasty short is to have W lose in 2000, and there are a number of spots to do it. McCain wins the nomination after the Bush camp's "black baby" gambit backfires. Gore wins Florida if Clinton handles Elian Gonzalez better. Or my favorite - W decides not to run after a heart-to-heart with his father's good friend, the sitting president, W's dear old Uncle Bill, who's in bad health from all the stress of the Lewinsky scandal.
W talks Jeb out of a future run, and Jeb doesn't have the same abilities as W anyway. So the GOP becomes the party of guys like McCain and Romney, and the religious factions end up feelin underserved.
As for Hillary...let's say she divorces Bill. She goes to the US Senate and wins on her own, and she decides to stay, seeing what the Presidency did to her marriage.
This way, HW is ousted in '92, Bill serves until 2000, W loses to McCain, who serves until '08. Maybe America takes a chance on Obama in '08. He almost got the nomination, but everyone agrees his future is in the Senate though I have heard his name bandied about for federal judge roles.
As for now, if not Jeb, I'm thinking Governor John Kasich out of Ohio.