Don't over estimate the the negatives of a choice being too obvious. For a first expansion, it is actually a huge plus.
It is true Egyptians are the obvious choice...so what? Norse and Greeks were as well, but they are the most relatable.
Egyptians have the benefit of both being familiar to players, yet have lore that is different enough from the current two to introduce new unique mechanics which in turn warrants their own expansion.
When I hear about the "non-obvious" ones I yawn, not because they are boring...well, actually, yes, because they are boring! I want some death gods! Some foreign (desert) tiles! I know these are totally unlike the current ones. That already puts Egyptians above the other potential factions, where in terms of mythology I really don't know one way or another.
I want something I know enough about to know it will be very different from the current two factions.