sounds like a fun question indeed.
Pick a topic you are very familiar with, and equally important, one you enjoy talking about.
Given your environmental background, a topic related to sustainability, renewables etc, maybe in conjunction with emerging markets (→ increases likelyhood of market sizing / market entry cases), could be a natural fit?
Now, having said that, do not try to overthink the topic - in either case you will still need to come well prepared with a robust, and repeatable approach to solving case interviews. Having a slight advantage by influencing the topic by itself will not be enough to succeed, as interviewers are very adept in telling apart raw case performance from impact of familiarity / experience with a topic.
If anything, too much familiarity with a topic can sometimes impair case performance, as candidates tend to deviate from using a well-structured approach they would apply when not familiar with the case / industry. Please be cognizant of that and avoid the pitfall! Make sure you come to the interview with an open mind and prepared for anything.
Hope this helps - wish you all the best for the interview! Do reach out, if you haver further questions.