I would like to see items, like spell books that while attuned offer the holder an extended spell list. 

I second this.

Currently the closest equivalent are the Tasha's Cauldron of Everything books but it's not the same and even they don't fully work as they're supposed to.

Essentially it would be nice to have a magic item with the same feature as Spells of the Mark for Dragonmark Races or Guild Spells for the Ravnica backgrounds. This way you could literally create books that characters could add and then be able to prepare/add those spells even if they're not normally for that character's class. This would be fantastic, true-to-setting way for Wildemount wizards to get dunamancy spells without needing to be one of those subclasses, as an example. 

This would be an excellent addition and save on all that nonsense of having to homebrew spells/backgrounds to add them.