I wouldn't even bother with the "save the League" stuff. That's a common enough theme in Titans and Nightwing history. If I could I'd ignore the League completely, but to ensure the stakes of an Event seem high enough you *have* to use them in some fashion.....so instead of saving them, I'd have Dick take some of them down. Show us that Bruce isn't the only baseline human who can pull down a god.
As for the hook? You've got a few options, and none of them are crazy great. You can pick a threat from within Dick's solo mythos, which basically limits you to Blockbuster and the Bludhaven mobs and Raptor. It'd take a lot of work to make them seem like a big enough problem to warrant a Event. Or you can grab a threat that Dick has co-ownership of, like the Court of Owls or Spyral. Arguably the best approach here, and one would assume that the Event would shift the villain into Dick's rogues gallery completely. Or you bring in a brand new threat nobody has heard of. Not a bad way to go but the lack of precedent will have to be balanced out carefully, so you don't look like you're trying too hard (avoid red shirt-ing someone, yknow?). Last option, you take a established threat Dick has no prior connection to, like Professor Zoom....but that just looks random and weird.
It it were me? I have an idea in my head for the Flamebird and Nightwing of Kryptonian myth coming to earth and being curious about the humans using their names. I won't bore people with the details, but it's an example of using an IP that Dick is connected to (his name comes from Superman) but doesn't really own.