So, here's a bit farfetched theory. The story of TH16 will allude to the story of Momotaro.
In case you don't know (shame on you, you not-weeb!), Momotaro is a boy who was born from a giant peach. Then together with his companions a dog, a monkey, and a pheasant, journeyed to defeat a bunch of troublemaking oni.
Now, at the end of the HSiFS demo, Reimu thought about her previous opponents "a fairy, a yamanba, and a komainu", as if there's something linking them. Let's look at the bosses.
- Komano represents the dog, obviously.
- Nemuno represents the monkey, because a hag looks human, and human is somewhat closely related to a monkey
- Etenity represents the pheasant, because fairies have wings and can fly. Pheasants also have wings and can fly. ...I did say this is farfetched
So, later bosses may include an oni or a peach-girl. Or less likely, a surprise appearance by Ringo (in the story, Momotaro tamed the animals with dango).