A repeat of the Future Trunks Saga (Dragon Ball Super)'s ending.
Which, given how Zamasu was practically Xehanort 2.0, isn't completely unlikely -- to my dismay.
?
I imagine you're talking about them wanting to see if humanity is "worthy" or something similar.
Zamasu decided humanity wasn't worth observing because they were savages (because what happens on one planet happens everywhere else obv) and is trying to kill them all, period. He saw someone he thought inferior being stronger, and went mad.
Xehanort is still trying to see if people are worthy of the true light the legends spoke of, but he's primarily doing what he does for himself, out of curiosity. He doesn't want to balance mortal life, but light and darkness. He's also the opposite of an immortal God, in BbS the clock was ticking against him.
He saw someone he thought inferior being stronger, and decided to put him to use.
If it's about them having more than one "version" of themselves, that's a really common trope and the way they go about it (t-travel aside) it's different and for different reasons.
They're pretty different characters.
Also, the ending of the Zamasu Arc is just impossible for it happens on many premises Kingdom Hearts just has no way of setting.