My fun idea would be both of them dying (or nearly die) then both awakening a power, even if only for a short duration. Like Naruto awakening the So6P within him due to the tailed beasts' chakra in him (much like Yuske from yu-yu-hakusho awakened his demon transformation after dying/nearly dying. This power up would be temporary as well.
With Sasuke, he could gain an eye transformation. The manga hasn't hinted that Sasuke has Senju DNA. However, hashirama gave Sasuke some of his chakra. So since there is a difference between DNA (permanent) and chakra (temporary) I feel he could get the Rinnegan for essentially the duration of the battle thanks to his near death experience. As soon as that chakra is used up, he will loose the Rinnegan as well.