I can only see two ending for Sansui:
1. Become just like his master and walking the endless path of strength-seeking to the higher place.
=>1.1: He will leave everyone and pursuit his ideal alone
=>1.2: He will meet some immortals just like himself and they will walk this path together
=>1.3: He will live as a human in this life, seeing his loved ones die and after that go back to the eternal training
2. Forfeit his ideal, live and die as a human together with his loved ones.
I personally like 1.2 and I think Sansui will mostly go the route 1, after all he have trained for 500 years already, I think he is more close to a monster or a god than a normal man.