darkSpyro

So...just what do you need to buy to access every area of the game?

One Sensei of every Element, that's it. Adventure Packs too obv.

Now, to get everything everything(not just areas), that, make sure they're all from each battle class, and a creation crystal of every element for their sets.

To access everything you need a Sensei of each element. To get 100%, however, you'll need 10 Crystals with a different Battle Class on each... OOOOR you could find a copy of SSA 3DS, Giants 3DS or Giants for the Wii and reset your Starter Pack Crystal

If you want all the Creation Pieces... Good luck on that! That would require getting all 31 Senseis and 10 CCs to finish those sets

So only senseis can open elemental gates?

And...couldn't I unlock all the creation parts via the app?

You can unlock all creation parts through the app.

You can't transfer the parts from the app to the game without buying cards or figures.

The App has everything, even sensei parts(forgot to mention those). You can make a character with them and order without needing the Sensei or crystal, but it's a costly shortcut

To be more precise, what you need if you want to play every levels in the game are :
1) One Sensei of each element to unlock the Elemental Realms. They are basically short levels that contain 3 exclusive imaginators parts, treasure chests and the soulgem of a Skylanders from the same element. For example the Dark Realm will have Hood Sickle's Soulgem.
2) The two Adventure Packs and Crash/Cortex to unlock the Thumpin Wumpa Island level.

But if you also want to have all Imaginators parts you will need all Senseis and one Crystal of each elements. Though keep in mind that Senseis unlock only two exclusives parts that are based on their designs, usually one gear and a weapon, while Crystals unlock a whole set based on a theme. For example the Undead crystal unlocks the Werewolf set.

Having one Sensei of every Battle Class is very optional though as it only unlocks an attack for Imaginators of the same class.

There's the optimal list though, if you're not picky. All APs and the starter mean you have acess to Water,Life,Tech,Earth and Air, and Brawler,Ninja and Sorcerer, so you can go from there.