D&D 5E What previous edition books would you like to see re-done?

[Paraxis]

It is actually still available to buy, and due to its nature, doesn't really require a conversion effort. An interesting product for sure but not a good example of a needed conversion.

There were some broken things in that book, I remember putting halberd blades on my boots.

 

[Parmandur]

Deities & Demigods, as others have said, bit more as a campaign world builder (Here's how to do Greek D&D, here's how to do Egyptian D&D, etc.).

Manual of the Planes, as bot toolkit and Planescape setting book.

Savage Species: liked it, but would love a more mellow 5e approach.

Love the suggestion above for box sets for settings: allows you to have DM info and player info separated, and have a Lost Mines type adventure for each setting introduced.

Oriental Adventures, but with less exoticism; same for Arabian Adventures.