Welcome to planDisney, Hope! 
I’m so happy to see that you’ll be visiting The Most Magical Place on Earth at the end of this year! AND celebrating New Year’s Eve is truly magical at Disney World! Be sure to watch the 
Disney Parks Blog for updates as you near your trip, friend! 
You are so smart to be asking questions about capacity for Magic Kingdom on New Year’s Eve as this is historically one of the most popular days at Magic Kingdom! (EPCOT also tends to reach capacity on this day too.)
If you have a reservation for Magic Kingdom with your Annual Pass, enter Magic Kingdom, and then leave partway through the day to dine at a different location, you will not be permitted entrance back into Magic Kingdom if it has reached its capacity for the day. My recommendation would be to change your dining reservation to a restaurant within Magic Kingdom. I also recommend getting to Magic Kingdom as early as you can, as capacity has been reached as early as 10:00AM/11:00AM. (A notification will be sent through the 
My Disney Experience Mobile App when park capacity has been reached for each theme park.) Better to be safe than sorry if you’re wanting to enjoy the evening festivities. 
For your dining reservation, my family always loves 
Liberty Tree Tavern! This is a great location at Magic Kingdom to enjoy comfort food… and a lot of it! The dessert here, Oooey Gooey Toffee Cake, always blows my mind — and I enjoy it best with a cold glass of milk! 
If you have additional questions, I hope that you’ll visit us again here at planDisney soon! 
Allyn