I probably won't have time to get/play everything I want, plus I'm probably forgetting some major wishlist items here due to my lack of updated knowledge about release dates, but whatever:
1. Assassin's Creed II
Basically, the first game was one of the most promising AAA titles of this generation so far, and it introduced some pretty great new concepts (the story was cool and interestingly told, the environments were jaw-droppingly beautiful, and the fluidity of movement is among gaming's best)...but it fell short in execution (dull, repetitive side missions, flat-out retarded enemy A.I., stale combat, and a pretty short running time considering how often you're repeating the same actions over and over again). The sequel seems to be fixing everything that was wrong with the first game (you can swim!) and adding some cool new features as well.
2. Modern Warfare 2
Call of Duty 4: Modern Warfare is one of my favorite games of all time. $60 for more of that plus a multiplayer system that enhances the already near-perfect one of the last game while (hopefully) not trying to fix what isn't broken is definitely a good buy as far as I can tell. The only thing that has me a little worried is that the singler player campaign looks like it might take things a little over the top for my tastes. CoD 4's campaign was best when it was raw and visceral, dropping us into the conflict without dramatizing or glorifying it (the end of "Shock and Awe" will be talked about for many years to come, I'm sure of it), but the new game seems to be mostly eschewing that for big, flashy set-pieces. Still, I've got faith in Infinity Ward to give us a great experience.
3. Grand Theft Auto IV: The Lost and Damned and The Ballad of Gay Tony
I've been missing out on these DLC releases, so maybe the December break is the perfect time to check 'em out. And while I'm at it, there's some Virtual Console titles I've got my eye on as well.