The way I see it, if we're going to have Thargoid conflict on any sensible scale, wormholes are a requirement.
Their emergence could very well be an interesting event as described above (I really like that idea). And then, some time later, a wormhole opens, a load of explorers go through it...
"Oh, hai there, Thargoid fleet... "
The war begins. Events are driven by wormholes opening up around the galaxy. And the battles are focused around both sides of the hole, humans trying to fight back the offensive and (at some point) campaigns for offensives of our own through holes.
Intelligence missions can be held to find out the pattern so we can start to plan the likely location of future holes. And so on.