Okay, so for "rare" candy to be rare. The candy at houses will have to be "normal/regular" candy

What we can do is add a Giant Pumpkin Boss to spawn during Halloween, you can replace it with giant blob and change slimy spoils to "pumpkin spoils". You can trade those spoils with the witch to obtain rare candy.

Add a cat that runs around the map, which you need to catch with your bug net to give the witch her "missing" cat back. Make the spawn rate low. If you retrieve the cat and talk to witch, she will give you a "Golden" candy. The cat will be running around fast and will be hard to catch, especially with that server lag :p.

House collecting = regular candy
Giant Pumpkin spoils = rare candy, not sure how many pumpkin spoils should be needed to trade for a rare candy.
Cat = Golden candy.

Prizes?

Regular Candy Bag, Costs 10 candy: 5% chance to obtain a Halloween hat (Trick or Treat) (Average hats)

Rare Candy Bag, Costs 1 rare candy: 100% chance to obtain a rare Halloween hat

Super Rare Candy Bag, Costs 1 Golden Candy: 75% chance to obtain rare Halloween hat, 25% chance to obtain a Halloween mount (A good one...)

Edit!

Make Zombie baddies, and Pirate baddies have a 1% rare candy drop rate.