Actually, I have to disagree about Kellar's Keep and Return of the Witch Lord. I think either can be played directly after the Game System, before the other. The story makes more sense if you play KK first, but Milton Bradley does seem to have made a reasonable attempt at making them independent of one another.
Mini-essay coming up!
Evidence:
1. Both the North American and European English Game System questbooks end by describing two future dangers--the trapped Emperor's army and the Witch Lord's return. (Admittedly it does spend more time on the former than the latter.)
2. There's no mention of the Witch Lord in the Kellar's Keep questbook that I can find (NA or EU), and no mention of the trapped Emperor's army in the Witch Lord questbook. Both conclude by congratulating the Heroes and making vague references to 'the forces of Chaos' and other future threats (and Skulmar, because anyone called Skulmar deserves to be mentioned as often as possible). If the two quest packs had a defined chronology, I'd expect to see Mentor telling the Heroes to go beat the Witch Lord at the end of KK, and congratulating them for saving the Emperor's army at the start of RotWL.
3. On the back of the KK and RotWL boxes (at least in the EU edition),
both threats are mentioned as if they're happening at the same time, and you can tackle either one first.
The EU KK box says the following (quoting from the Inn's pages): "The adventure continues... Beneath the Black Mountains the Witch Lord broods deeply, rekindling the embers of his malevolent ambitions. Soon he will have recovered enough strength to lead his skeleton army once more. Yet a greater danger looms, for the Emperor and the remnants of his army are still besieged in Karak Varn. Time is running out for them..." (etc)
And the EU RotWL box says the following: "The Emperor and the remnants of his army are besieged in Karak Varn. Time is running out for them, yet a greater evil must be thwarted. The Witch Lord lives. He broods deeply beneath the Black Mountains, rekindling the embers of his malevolent ambitions..." (etc)
To me, this suggests that either quest pack could be played after the Game System.
However...
...there's some evidence that KK comes before RotWL too. For one thing, the Game System questbook spends a lot more time on the Emperor's army than on the Witch Lord threat, emphasising that time is running out for them. Also, RotWL ends with the Emperor honouring you at the Palace, which would be a bit tricky for him to do if he's still holed up in Karak Varn...
So despite what I've said above, I suspect that in the designers' minds, KK came before RotWL in the narrative. But I reckon they decided at some point to make things more ambiguous--they just didn't do a thorough job of it.
Possibly this was for marketing reasons, so that people could buy either quest pack if they could only afford one. That way they'd presumably sell equal numbers of both. Otherwise RotWL might be almost as rare as Ogre Horde...
...you know, I might try this next time I get the chance to play HQ. I've never actually made it through RotWL because KK is so gruelling. Maybe I'll let the players decide which quest pack to tackle first. This will have consequences for the other quest pack, making it harder because they were too busy elsewhere. Hmm...