Demetrios1453
Hero
Durnan's guide to Undermountain, perhaps?
I do like the Nezram's Guide to the Planes angle, also Drizzt's Guide to Icewind Dale (or the North)...
I would LOVE to see a rehash of the Damara/Vaasa region, coupled with a rehash of the "Throne of Bloodstone" series, as an intro to high-level play (a "Mythic-Level Play book," if you will.) However, that might be a bit too far-fetched to gain a large audience.
Between SCAG and SKT, the North has been pretty well covered (other than possibly going a bit more in-depth with Waterdeep). And OotA did a good job of covering the northern Underdark as well. So I'm not sure what sort of product they could use his name on...