I know i am late to the convo but hear me out please, an identify any mistakes i make.
If i am not mistaken, Naruto and his children and the final Uzumaki's left, and to my knowledge there are no other senju's after maybe tsunade. Sasuke and his child(ren) are also the last Uchiha, so if the next Ashura Reincarnate will be a decendant of boruto and the next Indra Reincarnate will be a decendant of Sarada, if they both (Boruto and Sarada) have a child, grandchild etc, won't one of the later decendants, after Naruto and Sasuke die be the next both indra and ashura reincarnates, so since then they may be able to awaken the rinnegan as a result due to the mixing the genes.
A Few assunmtions i made:
Himawari does not have any children or the child is not an ashura reincarnate,
Karin Uzumaki did not have any children or the child is not an ashura reincarnate,
Sasuke and Naruto's decendants are the last of their respective clans,
This child (the one i say will have the rinnegan) will have a sharingan in the first place, so cannot activate the rinnegan (or maybe they can idk).
Boruto and Sarada and all other decendants only have one child so that indra and ashura reinncarnate in the same body,
Further more, i think that the child/ children will be like the Kaguya clan, where once in a while, a child is born with the kekkei genkai, such as kimimaru, so this child will have a rinnegan, then after a while after the death of this child/person, the next reincante will have a rinnegan, skipping a few generations b/c they are not indra ashura reincarnates.