As for Yokai Servants, there's plenty of potential for what you could do for them. On one hand, making sheets based purely on the yokai could work pretty well. But on the other, I think it would be interesting if Toriyama were to somehow bestow four of his best minions with the Saint Graphs of Raikou's four retainers a la the Fairy Knights of the Round Table.

And speaking of Raikou, she would definitely fit as a major ally in this Lostbelt.