Costumes
We don’t know.
ANet have provided different ‘hats’ each year that were unattainable for a while in GW1, but then eventually you could get them all as years went by, but they were quest rewards. When they added outfits in GW1, I believe you could buy them all year round.
If I had to guess, I’d say that they’ll re-sell the Mad King Thorn and Witch Costumes and then add more on top of that next year, but again – that is just a guess.