NASA has lost its cojones after Apollo got cancelled. They should have then immediately pushed for a long distance Mars mission.
NASA actually did
propose a Mars mission after Apollo. But Nixon wasn't a fan of NASA -- he liked the astronauts as a propaganda point, but that was it -- and when presented with the shuttle plan, he approved that only
because it didn't dismantle the space program. (Here's a good summary
.) There's an alt-history for you -- put another occupant in the White House in January 1969. Stephen Baxter gets to a manned Mars mission in his alt-history Voyage
, but it's a one-shot deal, the last gasp of an agency that's dead-ended itself.
What's going to get NASA back in the manned space game is China; a new space race is what's needed to spur the United States toward space exploration again. China makes serious moves to put a man on the moon, and NASA's back in the game, with politicians falling over each other to throw money at NASA.