They don't have to be all powerful, they could just gain a combination of elements (I suggested this in Post 6), or, if you prefer them being all powerful, then just let the toa use this ability once. It would take away the "form a kaita every time" problem, and give the toa reason to work out different/better sollutions. It's kindof like a one-time-use-only Nova Blast.
Personally, I could see G3 work with or without the Kaita. I could see Lego using it as a "creativity" or "Unity" selling point, especially if this G3 got a TV show.