CSX and CPKC have Christmas Holiday Trains that they use for charity work and special runs during the holiday season. The trains are decorated with Christmas lights and special liveries. We have the CSX CW44AH/CM44AH in game, which would be perfect for a Christmas on Sand Patch Grade DLC Pack. We would just need a CPKC route with their Holiday Trains and AC4400CWMs. Then we would need the special-painted rolling stock with lights on them too. Then we have the Polar Express. Several museums and tourist railroads (like the Grand Canyon RR) do annual Polar Express special trains during Christmas time. Of course, the real Polar Express, Pere Marquette Berkshire #1225, which is in operation on excursion trains, could also be added to the game in both PM and Polar Express liveries, along with some matching excursion passenger cars for the Polar Express and regular excursions as well.
Polar Express would probably run into licensing issues but given companies like SCS lay on their Winter Wonderland event every year, I’m surprised DTG haven’t looked at doing something like that or along the lines you suggest.