look in the december comic, (the latest one with kopaka nuva and tahu nuva face to face ready to kill each other) it
shows that werid cube and in the backround it shows the exo toa (its kinda hard to see) and when you look in the back of the exo toa intsuction manual, when you buy the exo toa, it shows tahu regular with no sword, and tahu nuva without armor or anything and it shows how to put them in the exo toa. so i think since the toa nuva have no elemental powers, they'll get the exo toa, put it on and get the bohrok forces and fight the bohrok kal. i also think the bohrok kal are just cahdok and gahdok. that they were soaked in protodermis too and like the toa kaita, they fell apart and are here to seek revenge.
'corse thats my theory.