Banjo & Kazooie don't really do anything terribly out of the ordinary. The most unique aspect of them, Wonderwing and it's limited uses, is basically a tweaked version of Robin's durability. Yggdrasil's Altar is also pretty standard besides the occasional Mimic.
I don't see why the transformations need to be invincible as their selling point, or be specials unto themselves for the most part. We have a couple of characters using something akin to stance systems (
) that effect either the entire character or just their stats, but we don't have something in between that transforms certain parts of your moveset and maybe stats or other actions while active without completely overhauling the character you're currently playing as.