NiceSo, going to echo a few things I have seen upthread, but what I wish for ...
5E Monster Manual II - I like Volo's and Mordenkainen's guides, but a pure monster book would be greatly appreciated. I am sure I could dig up at least 15 - 20 monsters from previous editions that could fit into 5E nicely that are not yet updated. Yes, I can do it, but life is busy and I would prefer to spend my time doing some world-building and plot development rather than figuring out stats.
5E Deities & Demigods - Maybe not quite the full 1E reproduction/treatment, but a good book that has the main fictional pantheons (Eberron, Forgotten Realms, Greyhawk, Krynn) plus maybe 3 or 4 fictionalized real-world pantheons (Egyptian, Greek, Mesopotamian & Norse are likely the least problematic in terms of cultural appropriation claims, might get away with Celtic). Take all of those and provide say 1 page per deity, with some baseline ideas on dogma, holy texts, holy days, major servants, basic mythology. On top of that, you add an intro on how to integrate fictionalized religion into a campaign, with perhaps some campaign hooks that could link into multiple pantheons presented in the books. Basically, expand on the brief section on religion in the DMG.
Adventure Map Folios/Packs - Give us one for each of the adventure books they create. Higher quality than some of the maps in the books. They do not even have to be poster size in all cases, as I can use 8.5 x 11 map sheets for certain smaller encounters.
Greyhawk Campaign Setting - No nostalgia here as I never owned any of the previous boxed sets/versions. I was around in the 80s heyday, but home brew and Known World was where things were at. I still home brew, but everything I have read about Greyhawk has inspired ideas. I would disagree with some of Gygax's ideas (some of the racial groups are a bit problematic now when it comes to current cultural understanding), but there is a lot to loot in terms of inspiration. No further support is needed either. Create a version with a nice poster map (or even better, a huge map to buy separately) and let people take the nostalgia, or a new place to adventure, and then move on to other things. DM's Guild could provide all further support.
Cheers
I would want a deities and demigods for 5E only if they brought back specialty priests for the different gods.