This is how sasuke is gonna get rinnegan. VERY SIMPLE
Sasuke is going to get hurt very soon.
Maybe by fighting juubito or getting hit while protecting naruto.
Thats when jugo will come in. He will do the same thing he did when sasuke got hurt fighting bee.
But this time sasuke will gain senju dna. Because jugo absorbed zetsu during the revival of the 4hokages.
Yes we dont see him actually absorbing zetsu, however on the first page of chapter 618 zetsu's body is no where to be found... so maybe thats what really happened, and we will see a flashback to him absorbing it after the confusion of awakening the rinnegan.
WHAT IS NEEDED TO AWAKEN RINNEGAN?
EMS + senju dna + near death experience..
Well while sasuke is hurt and he feels he is about to die, he starts realizing he's failed and just when he thinks its all over.
Jugo is done giving him dna. AND WALA...