One of the big factors is that all of my children and grandchildren will be up here for Christmas--which has not been the case in a few years. Having young ones around puts a few limitations on any series of movies that I would watch throughout the day. I had been thinking about doing a Guardians of the Galaxy marathon, but I think it would be both a bit long (as the last two Avengers movie and at least part of a Thor movie need to be included) and a bit too scary--especially in certain parts. On the opposite side, I've long considered doing a 1990s Disney musical movie marathon starting with The Little Mermaid (technically, it's a 1989 movie) and ending with Tarzan (which isn't really a musical, but Phil Collins' songs are absolutely on fire). There are some scary parts there, too, but my larger concern is being able to handle that much animation in one sitting (I haven't run the numbers just yet, but that's nine movies--which means somewhere between 14 to 18 hours).
Meanwhile, this birthday is when I hit the half-century mark, so I would like it to be special. This could mean going back to the movie series that started these marathons: the original Star Wars trilogy (which I first watched as a full marathon for my 18th birthday) or perhaps The Lord of the Rings (extended) trilogy which was the first birthday movie marathon from my current trend of birthday movie marathons (and which happened to be 20 years ago this year). The problem is that I've done both of them within the past five years (twice for Star Wars in the past 8 years). Ten years ago, I did the Pirates of the Caribbean movies, but has to be willing to watch a clear drop in quality after the first one (although, I do like how they tied it all back around at the end of the 5th one).
Clearly, I've given this some thought; although, I have--as of yet--no conclusions.