I'm assuming Jaina will have something to do with the bridge to next expansion, but I can't guess exactly how. Even Legion, probably the smoothest connection between expansions, had a lot of issues in what things were supposed to have happened in what order and what was the actual event that first tipped us off. I mean, we got that Son of the Wolf comic before Varian even died, let alone Anduin getting his sword.
In other words, I'm thinking that Jaina could come talk to us in a patch or a cutscene or even a comic, but at a point where we've already gotten the premise from the expansion's announcement, and later on we find out that canonically it was something else that first exposed us to the next conflict. That sounds like a semantic argument, but it's holding me back from guessing because depending on what next expansion is actually about, I think we could have some very different reasons for getting involved.
To be more specific, I feel like we might just have a break after 7.3 and then just get attacked in some pre-patch (that we'll see coming because of what's announced with the expansion), but we might also get a warning from Jaina before that.