When the calendar became base game, I was thinking that it meant they were working on lunar cycles and therefore werewolves. But that was the cottage living patch, so long ago. Maybe it didn't have any to do with werewolves. I do expect that we will be able to see the moon phase in the calendar.
If we can change the moon phase, I expect this to be possible for any player with the new pack, not just seasons owners. I hope we can also set the length of the lunar cycle in the options, similar to choosing the length of the season.
I like the ideas posted so far and hope that the lunar cycle (if we get one) will have effects on other Sims as well.