it might sound boring but I really enjoy anatomicae challenges that focuses on traditional - even academical - human anatomy/virtual life drawing studies.
I prefer human anatomy (since I never seem to find/take enough time to really nailing that on my own), but animal anatomy is great practice also.
Traditional anatomy studies and even master studies, that's my cup of tea at the moment and what will make a challenge really hard to resist.
Perhaps rendering the human figure in styles of various -isms/artists would be a fun and good practice for some future challenges?
Thanks for running the challenges, have fond memories of the ones I've participated in and looking forward to taking part in many more...