I'm new to the forums so excuse the ramblings that follow - I thought I would just toss out a few quick ideas...

The band of knights going after the Dragon isn't such a silly idea. Personally I would play around with the concept a little though. Perhaps an ancient order of paladins who require the dragon's soul to forge a [insert favorite magical trinket here] which they need to slay something more powerful than the dracolich? Perhaps the thing that brought it back from the Black Gates of Death? And the PCs just thwarted that by killing the Dragon first? So what is going to happen without the Magical thingy? And how are the PCs going to fix it?

As for other threats? I would go with the time honored back-stabbing dragon lieutenant. The one that secretly wants the Dracolich's power but is pretending to be his loyal servant while all the time aiding the PCs in the background. The PCs know the bastard is utterly evil and they shouldn't trust him but without his help the Dracolich could win

I like the idea of the Dragon Cult too, nothing keeps the PCs on their toes like feeling like they are being watched... ALL THE TIME! And because you know that the NPC -the one who the party all loves is a member of the Dragon Cult! Maybe even the King or Queen or one of the Royal children is a cult member? Maybe one of the PCs was once a Cult Member but is afraid to admit it to his fellow adventurers?

What if the Dragon eating all these ghosts is messing with the natural order of Death? What if things just aren't dying when they are supposed to because Death is too busy trying to repair the damage between worlds? Imagine the PCs surprise when their hated foe Trog-mar-Krell the Ogre King keeps getting back up after dying? What if a PC suffers a fatal wound that does not kill them but they know it should have? No Death, No explanations -I know my players would begin to freak out over that...

Anyway just a few ideas.