to me i think that sasuke will eventually get his rinnengan maybe before the war comes to an end. According to the research i did in naruto wikipedia and some cross checking of the manga.i noticed something. the stages of the sharingan is simply in four stages that is after the sharingan has acquired its full form. these are the actually stages
1. sharingan
2. Mangekyo sharingan
3.Eternal Mangekyo sharingan
4.Rinnegan
and since sasuke is a direct descendant of the elder son of the sage of six paths who had the rinnengan.i can strongly say he is bound to unlock it anytime soon. Also madara in his talk with the five kages at their summit he said the uchihas had a tablet written by the sage of the six paths himself which can only be read if you have the sharingan, and it became more clearer if you had mangekyo sharingan and rinnengan. In other words by all means sasuke will unlock the rinnengan because that is the final stage of his sharingan.