The first that came into fruition is the national stage, as we all know, is their MSBY vs. Schweiden Adlers match.

The second one that we thought is the world stage is the Olympics. The Olympics is considered the world stage of athletes out there as we know, so this would be also considered as the world stage.

But let's not forget that this promise (in chapter 37) is originally not a promise of them playing together, but against each other, and that has been made clear since the beginning.

So, the world stage Kageyama was talking about isn't exactly the Olympics, but Foreign League. Below is the last spread of the whole manga where they face each other off again... being a part of foreign leagues...

With this open ending, we now know that whoever Hinata faces off, at the very end, Kageyama will ALWAYS be the final boss