This is just a hot idea that flashed in my head but I want to share the idea.
Most of us assume Sasuke acquired his Rinnegan due to the presence of Ashura and Indra Chakra present in his body. But I remembered. Sasuke had another Chakra within his body.
Sasuke had a small remnant of Naruto and Kurama's Jinchuriki Chakra deep within them. Despite the cloak vanishing he should still have had an ember of Kurama Chakra deep within him.
Could it be possible that Kurama's Chakra remnant was mixed with Ashura's and Indra's Chakra to manifest Sasuke's unique Rinnegan?