The Sword of Summer question

so far he's written about Greek, roman,and Egyptian mythologies. Now he makes known his intentions to write about Norse myth. We know that Magnus is of relation to Anabeth which gives me the notion that he plans to combine all four of these mythologies in a 5-7 book collection of Harry Potter sized novels. they say the more the merrier but wouldn't it have been easier to leave out the Norse gods. The HoO series has already turned off some of his original Percy Jackson fans, It seems like a risky move to introduce an entirely new franchise which might not appeal to the bulk of his fan-base.
I can kind of see why he decided to add this new group of heroes. There isn't any real substantial threat left in the Greek/Roman or Egyptian worlds, with the exception of Setne. He has explained how Setne has stirred up the unrest between the Greek/Roman side and the Egyptian side, what he hasn't explained is how the world has almost ended three times, once in each series, but neither side knew about it or came to investigate, or is the MIST really that strong