My campaign that I've been running for the last ~2 years just ended, and my players intimated to me that they'd like to come back to my homebrew world with their characters for a sequel campaign (after an appropriately long break) so they can experience the wonders of high level play. The previous campaign ended at lv10.

I have some inklings if a plot, and I know one of the hallmarks of upper-tier play is opening up the world to crazy planar shenanigans, but I was wondering if anyone had any other tidbits of advice for a compelling higher- level campaign? 

I think I've decided on mind-flayers as the main baddies as I have some lore teasing that they had once visited my game world only to leave with an ancient empire's worth of slaves, so it'd be maybe structured as an alien invasion narrative as they return to invade the plane. 

I'm also heavily considering getting Matt Coville's Strongholds & Followers book since I like the idea of the players having a customizable home base, as well as becoming an established presence on the political landscape after saving the world in the first campaign. Maybe they'll also have to politic with powerful forces and make alliances with mythical beings to defeat the invaders. I have an idea of a secret illuminati of immortal beings (a vampire, a litch, an ancient dragon, a rich mage with stacks on stacks of Clone spell, etc) that secretly run the world that maybe they have to decide whether to get help from or overthrow, that kind of thing. 

Anyways I'm probably rambling, getting a lot of half-formed ideas whirling around here. What do ya think? 

Strongholds and Followers is awesome.

However, be aware that it is really only half the story. It provides the mechanism for building the stronghold(s) and how to run them day-to-day. But the political stuff about how the estate of the heroes interacts with the rest of the world will be published in a 2nd book, called Kingdoms and Warfare.