Hi Heather,
Love that you are planning ahead because January will be here before you know it!
There are some evenings when parks may hold their nighttime displays at the same time the park is scheduled to close. Illuminations is a great example of this - its typically held at 9 p.m. and Epcot usually closes at 9 p.m. unless Extra Magic Hours are scheduled. Its a great way to end the evening.
Generally, if you are already in line for a ride at the time the park closes, you will still be able to ride the attraction but they usually close off the lines at the time the park closes so no one else can get in line.
At the Magic Kingdom, the shops along Main Street will remain open for awhile following the fireworks. I usually take advantage of that and do some shopping because everyone will be exiting the park at the same time. I'd rather spend my time in the shops finding more souvenEARs than waiting with everyone else in line for the buses, monorails and ferry boats.
Please let us know if you have additional questions during your planning. Wishing you many magical memories :-) Allison