[Autumn]

Custom Tome spells cannot gain a domain to them, which disallows them from interacting with certain features, such as cheapening the FP Costs or interacting with other things.

I would like to suggest adding seeds that give a domain to a spell, either for the Extra-slot or for a small-seed type, sold somewhere in the world.

[Snake]

Maybe a Quality tool could be it too? Something that you can only have one of a type, and if you add another it replaces that specific quality.

[InsainArcaneBirdbrain]

Another simple idea that could rectify this is just defaulting the domain according to the base element of the tome(i.e. Dark custome would be Huggesoan while the wind one is Sylphid). Perhaps from there, an optional extra slot seed could be used to modify the domain away from the default if so desired.