Henry Carstens presents "unorthodox" ideas for strategy creation.

henrycarstens.com/101-trading-ideas/
(even a look inside the book on amazon is interesting)

Some more in his webinar:
https://www.youtube.com/watch?time_continue=420&v=zLAXxjXxxiQ

If brainstorming and thinking feels difficult, here's a systematic approach (easy reading, no worries)
https://www.farnamstreetblog.com/mental-models/