I'm great with coming up with new ideas. After reading your prompt my mind is already running. I used to do word-searches all the time when I was a kid, so I know the sort of words you're after. A varied mix of long and short words with majority being more obvious ones, with a few left field ones to keep them on their toes. Like off the top of my head, the more standard ones for a topic 'Trick or treat' might be:
Sweets
Candy apple
Trick
Treat
Costume
Chocolate bar
Though there are more left field ones like:
Door
Knock
Twilight
Share
Greeting
Surprise
Just as your list had things like mail man and Ghostbuster alongside werewolf and vampire. The list also illustrates what I said, with small words like orc up to long words like Frankenstein, and only a few two-word answers as exceptions. In this case, because they add humour for the adults. The reasons for the word size variation is because there are only so many letters available in a word-search, so they can't all be long words, otherwise they might not fit in so easily, and it would be quite a chaotic looking word-search by the end.
Another advantage I have is that I've worked at a supermarket and merchandised at least 6 years of Halloween sweets, costumes and decorations, so I intimately know what imagery and icons are associated with the holiday. I'll admit we don't celebrate Thanksgiving in the UK, but I get the gist from watching US dramas.
It seems like a fun project, so hope you pick my bid!