The adventure pack to me is what would be the bread and butter. Use the base CRPG engine and then create a whole new adventure, maybe one set in Waterdeep, Neverwinter or even Athkatla. You could move off the Sword Coast and do something Cormyr, the Dalelands, Anauroch or any of the other places within the Realms. Even leave the realms and use other DnD based game worlds.
You could argue that's what's happened back then with the (A)D&DGold Box games (SSI) to a degree. The engine itself and tools were barely ever much changed. That allowed them to churn out new adventures fairly quickly... never before nor after had there ever been so many D&D games released. There's still user made content being released to this day via the Unlimited Adventures toolset sold back then.
http://frua.rosedragon.org/The flipside to this approach is that... I mean, it's really story DLC isn't it if you break it down to modern terms? Even if you don't market it as such, say by selling each campaign as standalones at a lower price, that's how it would likely be perceived. So typically without a major update/new release, interest eventually declines. See also currently Solasta, which has had two fairly beefy story DLC (including features and art assets not at all in the base game), both of which not near as popular as the main release. So it's usually really just players who want more of the same experience coming back. Personally, I like the idea. If you're invested in a game, you want more. Still eventually, it's not commercially sustainable anymore to further support.
