I think as long as you have novel stories and don't string them out to long you can do formulaic writing and make it interesting, I still enjoy Koontz from time to time when he does his off beat stories (still love Frankstein and Odd Thomas series). Brandon has got the talent to make different but still formulaic stuff too, like I enjoy the world of Way of the Kings enough to have read the books published so far, but some of his other series I never got into because it felt a bit to similar.
Honestly Butcher when he was in his prime of pushing two books a year I still loved, as The Dresden Files was such a fun concept of modern day wizard and yet I still enjoyed the Codex Alera greatly too because it was a different world concept.
I guess what I am saying is they want to do formulaic, I find I enjoy if they do a different style fantasy next to the high or normal fantasy series to keep it interesting.