The links between Gore and Richard Nixon in a niche in history has been discussed before. Both men lost their elections very narrowly, and their supporters contested the results. Both men went out of politics. Both lost to administrations that were popular in the aftermath of a national tragedy but became very unpopular due in large part to foreign wars that were fought on what turned out to be a false casus belli. I'm shoehorning in Nixon's case, as Kennedy and Johnson were separate administrations, but you get the idea.
In that niche of history, Al Gore, like Nixon in 1968, could have run in 2008. Many were expecting him to. He didn't.
To be honest, a portion of this scenario that has not been discussed is the fact that Nixon spent his time in the wilderness planning to be president, building connections and allies and making himself someone the Republicans could vote for. Al Gore did not do that, and instead very much went for a life after politics with no effort or intent to run for president later. And one of the bigger issues for him during his political retirement was the fact that, publicly, the perception of him increasingly grew to be an archetype of a Liberal, and Liberal in the four letter word sense. Had he had the intent to be president later in the wake of 2000, he would have kept at least some politician speak to not alienate the public in the run up to 2008. So a scenario where Gore runs in 2008 would involve Gore deciding early after 2000 that he is going to run again, and honing his character and efforts carefully to build up the infrastructure over the course of eight years to run in 2008.
What if Al Gore, with that early intention to run again, ran for the presidency in 2008? And what if he had won?