I've always enjoyed trying these, even if I may fall behind this November (I'm going to a convention at the end of November, and I'll be busy running a big game, so that may chew up writing time).
Ideas I've suggested previously but never got round to:
A vis source a day - always useful, and if I run out of good ideas I'm sure I can still come up with enough so-so ideas to finish the month.
Marcus of Paris' recruitment ledger - attempt a grog a day, with the framing story that they are known to the redcap Marcus (from The Lion & The Lily) who recruits staff for Normandy covenants.
A wealth creation scheme a day - I'm sure there must be at least 30 good ways to use magic to earn money.
I would enjoy any of these - and I'd actually be happy to do the covenant-a-day or vis-a-day if they'd be more interesting to people than familiar-a-day.
I've been thinking about doing a spirit a day or maybe a daimon a day - I've been looking at theurgy stuff for my own saga recently and would quite like to build up a 'daimonic bestiary' ala the appendix at the end of the Mysteries: Revised.
I often find that with something like theurgy it can be quite hard to go 'ok I can invoke a daimon, but what exactly do I want to do with that?' and having a collection of examples to flick through can help create those 'ooh, that would be a good hook for X!' ideas.
As a suggestion, any time you're doing something that requires significant math, such as a magical creature a day, or a fully-statted person a day, it is helpful to get some of the work done in advance. That's one of the things that always ruined me when I had my Apprentice a Day, I had to come up with an idea I really liked and then also to run the numbers on it, and then....
Yeah. I think I just skipped abilities on a few of them because I didn't have time.
I just want to thank everyone who is doing such amazing work in this forum. I am so impressed and inspired! I'm also stealing liberally for my own saga.