Elves would definitely be something that would be compatible with Tobias’s Norse expansion pack.
What kind of elves are you thinking of? Are they the cute little elves of Santa’s workshop, or something a little more stuffy like a Tolkein type of elf?
Considering that Stonehearth is going to have sentient rabbits, I think more traditional fairy creatures like elves would fit in just fine. Creating elves as a full expansion pack mod like Tobias is doing would definitely have some fans. It just will take some work.