I've always had a lot of fun (and a fair dose of frustration) pondering just where the Apollo landing sites would have been located had Apollo 13 not aborted and Apollos 15 and 19 not been canceled.
At the time Apollo 13 flew, landing sites for two of the remaining Apollos were relatively well locked in. Apollo 14 was going to a Littrow landing site roughly 40km west of the Apollo 17 Taurus-Littrow site. They would have landed near the edge of the dark mantling that extends out onto the lava floor of Mare Serenitatis, within walking distance of two distinct ground albedos and a wrinkle ridge. Samples from that Littrow site would have pinned down the dark mantling as the admixture of ancient dark (and orange!) volcanic glass from fire fountains into the regolith, and the Taurus-Littrow site would never have come up for later landings.
The other site that had been nailed down was Descartes for Apollo 16. The planners knew they needed the capabilities of a J mission for this site, and while they were still vacillating between two different sites, one just outside of Descartes' wrecked rim (the site that was eventually used) or another closer to the Kant Plateau, the planners had locked this site in for Apollo 16. The Apollo 13 and 14 Hycon cameras were designed to provide stereo coverage for site validation for the two finalists for the Descartes site, and the 13 abort put great pressure on getting acceptable coverage during 14.
So, with those two sites locked in, we are left with another H mission (Apollo 15) and three more J missions (Apollos 17, 18 and 19) for which we need to find landing sites.
Now, Apollo 14 went through a couple of landing site selection cycles, as its launch date changed. When it appeared that Apollo flights would proceed every four months, Apollo 14 was scheduled for a July, 1970 launch. Due to seasonal impacts on lunar trajectories, the Littrow site is not readily available in mid-summer, it becomes impossible to reach the correct orbit within the mass and propellant margins available to Apollo. So, for a July, 1970 mission, Apollo 14 was provisionally assigned a landing site about 500 meters to the west (IIRC) of the crater Censorinus. It's a moderately-fresh large crater (3.8km wide) with a very bright ejecta blanket, southeast of the Sea of tranquility, near the crater Maskelyne.
When Apollo 14 was pushed back to October, 1970 with the spreading out of missions to five-to-eight-month intervals, Littrow became available, and was assigned to Apollo 14.
So, let's assume Apollo 13 did not abort, and, for the sake of future crew selections, that Ken Mattingly was not scrubbed from the flight. In other words, let's take the unnatural drama away... *smile*...
Apollo 14 flies in October, 1970 to Littrow. As I mentioned above, it solves the riddle of the very dark soils seen from Earth and from orbit. Checks off that box, so landing on or near very dark soils becomes far less of a factor in future site considerations.
Since a great deal of work had been done on the Censorinus landing site, and since Censorinus is closer to the equator than Littrow and thus available for greater parts of the year, I've always figured that an H-mission Apollo 15 would perform the Censorinus mission in April or May of 1971. The EVA-2 on such a flight would certainly have returned some impressive pictures, taken from the very rim of a moderately fresh, nearly 4-km-wide crater.
That would lead us to a winter 1971 flight of Apollo 16 to Descartes, which is once again close enough to the equator to be available most all year 'round. This is the one mission that would have likely gone off pretty much as the one we all remember. Young and Duke on the plains of Descartes, albeit with Jack Swigert running the first SIM bay and making the first cislunar EVA.
The thing about this is that, if we're going to carry it out to the full complement of missions, the backup crew for this mission would be the crew for the final lunar landing, Apollo 19. Slayton's original backup crew for 16, when he thought there was a chance at an Apollo 19, was Fred Haise (CDR), Bill Pogue (CMP) and Gerry Carr (LMP). However, Slayton named that crew after Apollo 13 aborted, and Slayton had a rule -- no one got more than one lunar landing.
Slayton's basic rotation was that he considered CMPs to be second-in-command on the crew, trusted to run the CSM solo. They were commanders-in-training. The rotation took a CMP from one flight, made him the backup CDR three flights down the road, and prime CDR three flights later.
So, in Slayton's original rotation, Ken Mattingly would fly as CMP of Apollo 13, be the backup CDR of Apollo 16, and fly as CDR of Apollo 19.
So, it's my belief that had Apollo 13 not aborted, the final Apollo lunar expedition would have been manned by Mattingly-Pogue-Carr.
Back to the last three J missions. I think Hadley was irresistable, and would have been the target for Apollo 17 or Apollo 18. It depends on just how hard Jack Schmitt, who would have been the Apollo 18 LMP on Dick Gordon's crew, would have fought for Hadley over a somewhat less interesting sight like the Marius Hills. Considering the obviously volcanic nature of Marius, I'm pretty certain it would have taken the other spot.
Apollo 17 would have been Cernan-Evans-Engle, and would have been a summer or fall 1972 flight. After Apollo 17, Skylab would have flown. The hiatus would end with the flight of Apollo 18 in the spring of 1974, flown by the Gordon-Brand-Schmitt crew.
So, we have 17 and 18 going to Marius and Hadley, in one order or the other, bracketing the Skylab missions. We're left with 19.
The perennial choice for a climactic J mission was always Alphonsus. But it was *so* perennial that the site selection committee was getting tired of hearing about it. I think the extra J mission SIM bay work might have found some other really fascinating place. For my money, I think landing midway between the Flamsteed Ring and the Surveyor 1 landing site would have provided a great finish for Apollo. Flamsteed is original lunar crust, and the lavas at the Surveyor 1 site have been argued to be some of the youngest on the Moon. A true oldest-to-youngest mission, with the added fun of picking up more pieces of unmanned landers... So, my vote for Apollo 19 is a flight in the winter of 1974 to the Flamsteed-Storms site, flown by Mattingly-Pogue-Carr.
Anyway, that's some of what bounces around in this head of mine.
-the other Doug