I'd like to see the option for a longer game.
As an option, "never-as-the-default" type of thing, I philosophically have no objection. However...
12 months is often not enough to develop the whole colony if things go badly early.
Whaaa? Really? I've played a lot of games of Mule, and I've yet to ever see even one of my games end without the entire colony being developed. Usually they're pretty close to being completely developed around round 10.
At 18 or 24, the quality of land (crysite value) becomes important, as does keeping the food/energy supply at actual need instead of very high or very low.
I agree that lengthening the game would increase the importance of having land with good crystite. I would say that it would make it TOO important. Right now, the game is interesting because different things become valuable at different times. But by the 12th month, crystite is (generally) the best resource to be mass-producing. Smithore is usually not worth much by round 12 because all the plots are developed, so no more mules need to be made. Food and energy are only worth anything later on if there's some sort of disaster or player misstep, and even then they're only worth more for a round or two before the players adjust and recover. Not much about the above situation would change for any months past round 12... crystite would still be the way to go for every subsequent month.
So lengthening the game would, in my opinion, change the strategy to emphasize crystite plots way too much. You could safely ignore all the early month happenings with strategically getting into the other resources and at the right times and maximizing profits, because you'd know that none of that jockeying for position would matter in the long run... it'd be all about who could produce the most crystite at the end month after month.
Plus, the gameplay for the extra months would be quite boring. I envision players pretty much always running straight to the pub each turn, as they'd already have their plots all set up for maximum crystite production and adequate energy production, and there would be little reason to rearrange anything.