I'm curious about the extremes possible within each Koppen climate zone. What's the highest temperature recorded in a Cfb climate, or the most amount of snow recorded in one month in a Cwa climate, etc.? It might be hard to find definitive answers but fun to try. For example I'd bet the -31F recorded in Nowata, Oklahoma in February 2011 was one of the coldest on record for a humid subtropical climate. And there's a very good chance the coldest temperature for a tropical climate was recorded in Northeastern Mexico or South Florida.
We could also combine climate zones a little bit--like Cfb/Csb/Cwb all in one category for certain records, for example. Or not. Lots of possibilities to explore