There will be future updates. Currently there are no plans for the remaining spells, it will depend on any story ideas that are added later.
The King in Yellow and Hastur are a reference to the Cthulhu mythos stories as I am a bit of a fan of them, and that is why the 'Severn Valley' mod has more such references. In general the mythos is more horror that this game generally has, so it was more just about Monique's discomfort with spooky stuff.
I don't know how you're developing the last two spells, but if you do, I'd like them to include a blessing spell that helps me fight off demons at Sacred Clearing and one that can help me create mana stones without a blue box.
