Forgotten Realms Campaign Setting has been my favorite of the main setting books. If they can find some way to combine the flavor of the grey box and the production of the 3E book, it could be something special.
As an aside, I like Weapons of Legacy but I don't see why it needs an entire book. It seems like it would do well tucked into the DMG or an Unearthed Arcana.
Speaking of which, Unearthed Arcana, but I'd like to see it as a kind of "options gone wild" book - a kind of combination of the PHB and DMG, but an emphasis on "fringe" options.
What you describe of a new FRCS is exactly the sort of thing I'd want. The flavour of the 2E FR products, combined with the 3E production. I did like the format of the 3E FRCS as well.
Weapons of Legacy had a heap of cool weapon and magic item histories that made life easier for a DM. Even if I hate the system in the book, that fluff is still really good and completely usable. Between the fluff for created items and explaining rules of how to create an item yourself I think you could easily fill out a book.
I'd be keep to see an Unearthed Arcana again. That's one of my favourite 3.xE books. However, from what I have heard about the 5E DMG, it sounds like that may cover some of the territory Unearthered Arcana previously did.
The Encyclopedia Magica like in 2nd ed.
I love my Encyclopedia Magic set. However, many of the items in it were really unbalanced and sometimes the cost varied significantly for items that did basically the same thing but were published in different books (or in Dragon Magazine). I'd personally be happy with something similar to the 3E Magic Item Compendium. That book has a lot of fun magic items. Something like that, but even bigger, for 5E would be awesome.
Does wanting Dungeon magazine back count? Honestly, that's my one big wish, the rest I can bash together on my own.
I'd love to see Dungeon Magazine come back. I'd even accept just a digital version of it, so long as it was compiled into montly PDFs.
Alot of great suggestions but a few dozen people requesting new versions of old material might not be enough to convince WoTC to reprint stuff. Especially if the average DM is cheap and thinks outside of the proverbial box IE by just converting old material to his 5E campaign which is not too hard to do.
As for the OP request as to what I want to see reprinted, nothing really, I would prefer WoTC concentrated on new quality products instead.
I wasn't thinking of WotC re-printing products they've done before. I was thinking more along the lines of what books it would be cool for them to have another go at. In some cases, such as the Weapons of Legacy book, I'd want them to chuck out most of the stuff they did previously, but keep the core concept (weapons that get more powerful as your character does) and start from scratch.
In other cases, such as the World Builder's Guidebook, most of what they did was pretty awesome. However, it was made over 20 years ago, so it could do with some updating and expanding.