Module B4 - The Lost City, revisited and revised for 5e. One of the best of the early Basic/Expert modules. It was revisited in part during the latter days of 3.5e, but I'd love to see a really extensive treatment (all levels of the pyramid, the underground realm, and the catacombs beyond).
Also, for nostalgia's sake, one of the classic old AD&D series. The Giants-Drow-Queen series has been re-done multiple times, as have Ravenloft, Slavers, Temple of Elemental Evil, etc. So, let's go for either U1-3 (Sinister Secret of Saltmarsh, etc), or I3-5 (Desert of Desolation series). Either of those 3-module series could do with a serious 5e revamp. They're also two of the "classic" module series which have some fundamental flaws in the original 1e version, so it's not so easy to just convert them yourself. U2 has some structural problems due to the nature of the "enemy", and I3-5 suffer from some weird tone and balance issues (I5 particularly so).
Deities and Demigods, but focused more on legendary monsters, avatars, clerical domains, adventure hooks and themes for each of the mythologies (rather than "stats for gods").
The stronghold-building, dominion-running and mass-combat sections of the BECMI Rules Cyclopedia.
Manual of the Planes, or anything for Planescape, if done right. Which means: respecting and building on previous lore, without throwing it all away and going a different direction. For starters, I'd like to find out more about Zariel, and how she rolled Bel for lordship of Avernus (1st layer of Hell). Last I heard, Zariel was getting power-siphoned by Bel and was nearly toast. I suspect Tiamat's involvement, somehow.