When I attended the festival in Anaheim, there were two items I found I had to purchase: sunscreen, and a sweatshirt. The evenings were quite cool and windy!
Long airflights can also get quite chilly, so I would recommend a jacket or long-sleeved shirt to carry along in case you need it. This time, I'm hoping a light jacket will do the trick for both evenings and the flight, and a couple of long-sleeved shirts for air-conditioning.
Remember that most airlines are charging an additional $25 per checked suitcase each way.
As I recall, dress for the banquet is not exceedingly formal. I don't recall too many suits and ties, but someone please correct me if I'm wrong! While I admire the well-dressed attire of Barnabas and Quentin in the 1970s (suit and tie) and the era when people dressed up even when flying, I admit I like the comfort of somewhat more casual ware on all but the most formal occasions.