Monster Manual II for sure.
Maybe an Advanced Player's Handbook and Advanced DMG and/or a "Alternative Rules Tome" that reintroduces the idea of gritty mechanics and other options that are currently 2014 relics, or never made in to 5E
Also, a modern "Lords of Madness: Aberrations" guidebook, and a modern reimagining of the Liber Mortis.
Although I am sure more campaign adventure books are on the horizon, I am done with those and never benefit from them; I need stuff I as a homebrew/home campaign DM can use. I would make an exception for a hardcore Shadowfell revisitation akin to what was done for that plane in the 4E era, though. And it would be cool if they did a Nentir Vale setting book.