it's always a challenge to come up with new ideas and to execute them as either sculpts or illustrations. The most challenging project: the design/sculpting of three china pieces for ***** China, where I then worked as an employee, that commemorated an artist who worked for the company 100 years previously. The project's intent was not to imitate the artist but to do new items in the spirit/attitude that she may have used. The greatest compliment I received was that some employees of the company believed them to be originals of that artist.
Personally, the creation of a full picture book dummy, creating the story and illustrations and layout for the 32 page book is quite involving.
All fine art watercolor & drawings are always a challenge, especially if portrait work is required.