Rockstar Games is planning to release the returning Christmas DLC Content like they do every year. No GTA Online DLC will release this December, however, I see that they are planning to announce a new DLC this January or very late December. Very late December would be when the Tuneables get updated. Christmas content should be added on the 15th or 19th of December along with snow like they do every year.
I Simply do not see a DLC releasing this month. A DLC in January will most likely contain bonus content (Adversary Modes or In game unlocks) for a Valentines Day DLC.
Qbeta branch was also updated not too long ago. Typically in this instance, you would see a DLC a couple months after the update. If you were to apply this, we would land in the month of January.