Speaking of youkai and their need of human fear. I just think of a simple horrifying plot.
Suppose that some youkai had found a way to recreate Hourai Elixir without Kaguya's and Eirin's ability. Then, they captured several humans from the human village and imbued them with the elixir, making them unable to die. Now, even if the humans are immortal, they are still helpless against youkai. However, for youkai, immortal humans are essentially their free food supply of fear since youkai need humans alive in order for them to exist. Now, they can scare or even kill the humans without consequences since they will revive anyway. Then, the same youkai started to make a deal between several other youkai, promising them a free endless source of fear. Thus, many youkai joined them, breaking the whole spell card rules.
That would make an interesting plot if this was Canon.