The strongest hero.

Not necessarily, I think there will be two equally valid endings, like in Dark Messiah.

But the further plot needs to be based on something, or should it follow two parallel storylines? And in Dark Messiah, the story remained unfinished, and this conclusion should eventually come, with a clear interpretation.

And how about in Star Wars?

The player chooses between the Sith and the Jedi, and naturally, when playing as the Sith, they have to kill key characters in the finale, who then, as if nothing happened, decide the fate of the galaxy (Revan, Darth Vader). That is, the player is given a choice, but only one ending is recognized as canonical, and the further plot is built around it.