I think 17,21 is really the only theme of the crossword. Nearly all the answers represent people who are either examples of all of them (12a, 13a, 13d. 22a), examples of one of them (10a, 11a, 16d, 24a, 27d, and in a way, 20a)), or related to them (19a, 25d). I think you could even squeeze in 1d, 8d, and 23d, if you're willing to be more gender-fluid. I'm sure I've missed a few, but that's enough Googling for the night. It's probably the most educational crossword I've done.