AK does not have fireworks for New Years Eve, they still close early, but MGM, Epcot and MK will. Downtown Disney, in past has had fireworks at Pleasure Island (special ticketed event) but I have not heard what the plan is this year with all the construction. No, all parks do not reach capacity. MK usually does and VERY early, but you can usually get into Epcot as it has the largest capacity. A good idea, if you are going to be at WDW for more than just NYE... MK does the full NYE offerings on Dec 30 and Dec 31. If you go to MK on the 30th you will get to see the fireworks and have the experience and then go to Epcot or MGM on NYE.