Considering the importance of Kemmler, the WW1 angle might be fun. I suspect it would be very League of Extraordinary Gentlemen. In fact, a series spanning Kemmler's life (maybe not his POV) would be interesting. It would be good to see what happened in October 31, 1961.
Also, Hastings seems to be very important so it would be good to see that too.
Anything about Eb in his younger days, Maggie when she was stirring up a ruckus, the OG Merlin and Camelot, and particularly anything set in ancient times (Greece, Egypt etc).
But as Morris says, I probably would prefer him to just write more of the current series. I guess we'll see.