Most likely you will see most of the parks fill their capacity that day. I went in 1997 and did MGM on 12/31. A CM told me ahead of time to pick a park (already had), hopefully I had ADR's (which I did), plan on getting there as soon as it opened and not to plan to leave, unless I did not want to get back in to the park. By 10am the park had hit it's capacity. When this happens, they cannot let any more guests in until enough guests have left the park, due to safety and fire reasons. It was packed to the hilt. There were lines everywhere. When I found a spot near the stage (where the hat is now), I panned my camcorder around and all you saw was a sea of heads. The fireworks ended around 12:20am and by the time I got back to my resort, it was close to 2am. The lines for the buses were longer than any of the lines I saw in the park.
You are not guarenteed readmittance (or even admittance) if a park has reached it's capacity. Again, it is a fire and safety issue. If you have not already done so, I would make sure to make ADR's at every sit-down restaurant you want to dine at, as it is extremely busy that time of year. Can't tell you how many angry people I saw being turned away because the restaurant was full and the guest did not plan ahead of time.
If you leave the park for dinner, plan on not being able to get back in to the park. You can watch the fireworks from the beach of the Polynesian after dinner (depending on what time your ADR is for).