Kale, Broccoli, Brussels Sprouts, and Cabbage are All the Same Species
So does that mean that any one of the above can be used for this challenge? Because that does change things.
It´s not my place to decide, but if cauliflower, broccoli and sprouts are included, I reckon we´ll be opening up a totally different kettle of fish!!
It ultimately up to Timenspace to decide but my feeling is if it gets too broad a category, it becomes unfocussed as a challenge. Brassicas as mentioned in JAS_OH1 's link includes at least 22 vegetables such as turnips, radishes and watercress as well as cauliflower, broccoli and Brussels sprouts.
Brassica Vegetable Guide: 15 Types of Brassicas - 2022 - MasterClass
Timenspace, if you want to extend the challenge to include other types of brassicas its up to you.