So we have the Summer Court, Winter Court, Wyld Court and Rogue Court. Sounds like an unlimited source of material to me.
It also plays in to the whole concept that nothing is every really destroyed, it just changes. The Spring and Autumn Courts were broken (maybe by Summer and Winter - maybe something else) and were reborn as the Wyld and Rogue Courts.
'Circle of life grasshopper - circle of life.'
Don't forget "Santa Claus" and his 'Court' too. One power I would give the Christmas Knight would likely Supernatural Sense (-1 or perhaps -2) to automatically know who is naughty and who is nice.
Not sure what sort of 'weapon of office' I would have, if any. Possibly some form or lariat or similar, as could be used to spur/drive animals dragging a sleigh forward.
Another possible Item of Power to be used by the Knight serving the Erlking would be a hunting horn instead of a bow or spear. I can definately see the Erlking utilizing a spear himself, but his Knight is in service to the Court, so could be equipped differently.