I haven't had any motivation to work in the toolset, after pounding away at things for 2 years (Path of Evil), and then immediately another 2 years and a few months (Crimmor). Here are some ideas I've tossed around at various points. In no particular order.
A sequel to Path of Evil, with the player playing Tan, the tiefling companion from the first Path of Evil. The idea would be the PoE pc granted Tan's request, and now Tan is free to seek his revenge. This would be a high level campaign set in the Abyss (and possibly a short stay in Sigil to transition there, as the PoE stronghold has access to Sigil). Starting level would be 16-18. Instead of the open world and "blank" PC of PoE, the story would assume the player is Tan the Tiefling, so it would be more story focused than PoE. The PoE companion Carcarin the wizard also has unfinished business on the Lower Planes, so she may come along as a companion.
Merchant: The player would be a merchant trying to build and run a merchant company. This would take advantage of the customizable SoZ trade system I published. You'd probably start as a merchant apprentice, and work for a merchant company before striking out on your own.
Asmodeus successful: Official lore has Asmodeus repeatedly trying to drag Abeir-Toril into his plane of the Abyss, as part of plots to increase his own power. This campaign would start after Asmodeus has succeeded. Waterdeep, or some other suitably large power, would call Faerun's powers together to try to get Abeir-Toril back to the Prime, and provide portals to whatever locations were necessary to do so. You could easily have good and evil characters working towards this goal, since the evil Faerun powers would be facing existential threats from being in the Abyss (or being there is simply inconvenient to their own plans). Obviously this would be an epic level campaign as the most power people in Faerun would get called for the effort.
Generic adventuring without an overarching story, based out of Crimmor. This would let me reuse the Crimmor maps as the base, and this would resemble Wyrin's White Plume Mountain of different areas on an OM to have adventures.
Dark Sun: I'd love to do something here, but it's basically not doable as too much custom work would need to be done to do the setting justice.