Like the title says, does the world have to fill up with 100% owners first? if so, what happens if it never fills up?

Yes, world must fill. They always do, one way or another but it might take time.

I always say "Choose a world not a team. You can change your team but you can't change the world."

That was a merged world so two or more worlds had trouble filling and became one. Good news is that you only have one spot left and it rolled on 1/3. That's not too bad.