Been asked to run a very specific campaign and I don't know how to do it

So for starters, I love my players. They're the best, we have so much fun together and I had a blast DMing for them... but I was never expecting them to ask for a sequel.

Basically, over the course of the first campaign, their wild magic sorcerer/wild magic barbarian with the lucky feat caused enough random chaos to attract the attention of the Modron empire (in my world, Modrons are basically an empire that seeks to bring order to the universe through conquest), and they sent scouts to gauge the threat. My players LOVED this, and after the campaign ended they started clamoring for a sequel. In short, they want to play a campaign where their characters fight against the Modron empire in order to keep them from hounding after their teammate. The problem is, I don't really know how to do it.

For starters, they want me to run the campaign for their characters at level 20 (they'd just hit 15 at the end of the previous campaign), and I have absolutely no idea how to balance a campaign around four level 20 characters decked out with all sorts of magic weapons and items.

And then, I'm kind of stumped on how to get them to go against the Modrons. Their characters are great, but they're really bad at following plot hooks. For example, I had two ideas in the past, but the more I thought about them the more I realized they wouldn't work. The first one was that the Modrons simply attacked their hub city, which I think could work, but the party would need a way to follow the Modrons and they're really bad at that. The other idea, which is my favorite but even less likely to work, involves the Slaad, which in my world have been reduced to a couple of hidden settlements, and would worship the wild magic PC like a god. However, the problem there is that my players have a history of murdering people they don't trust, and would absolutely murder the Slaad if they approached them.
At this point, I'm kind of stumped. I want to do this for my players and I think it would be fun to DM as well, but I'm kind of backed into a corner.

I'd really appreciate some advice, both about encounter balancing for level 20 characters and about making a plot hook that my players won't ignore or kill.