darkSpyro

So I've read a bit about the game, but I've been trying to spoilers of any kind.
But I've heard there's no hats this year, right?
Which brings the question, what do adventure packs hold, other than more gamelay?

Also, I only need one sensei of each fighting style to unlock everything, right?
Just trying to plan out buying, since if APs don't hold anything and I don't really need another Brawler, I might wait on them till later.

The Adventure Packs hold the Soul Gems of the Senseis that come with the packs (Air Strike for GPO, Boom Bloom for EEF, Crash and Cortex for TWI).

Unlocks depends on what you mean by "everything". The most important unlock would be the Sensei Elemental Realms (short extra levels, which also have a few exclusive Imaginator pieces) which are unlocked depending on the Sensei's Element.

Every single Sensei also unlocks 1 exclusive Imaginator part, as well as a weapon (usually their own) and a Secret Techniques for Imaginators depending on Battle Class. Each Element of Crystals also unlock exclusive Imaginator pieces - one set of 5 pieces each.