Here are some of the niche cozy-mystery series themes I've seen so far:
- Knitting or Crocheting (as stated)
- Candy store
- Food/Restaurants in general
- Cheese shop
- Wine country
- Travel agent
- Crafts (soap-making and so on)
- High fashion
- Vintage clothing shop
- Card-making (another craft)
It reminds me of Barbie dolls. There's one for (nearly) every career or interest!
Personally, I think the card-maker mysteries seem the strangest of the bunch. I'm not sure why... It just feels so completely random.