Dyeing book covers: this might make it in at some point if we add different cover colors for lore books. It's not impossible, especially if we expand the lore available for Archaeology. There's already some lore that might not come in book form, so there will be new sprites anyway. I'm not sure about dyeing them; it wouldn't be too difficult to add since the new Godot engine takes care of a lot of dye-related stuff we used to have to do manually. It'd probably depend on what Pica wants to do with it.

Collating existing papers into a book: The Cookbook item is unique from the Book item because it's not sap-bound. That's what allows you to put pages in. (Technically, it shouldn't even really require a Book to craft, since you are ripping out all the pages except for the title page, but it was an easy shorthand. If we add book covers as a separate item, it's possible it would require one instead of a full Book.) If people use the book-writing system a lot, it's definitely possible we would expand it into a more robust binding system. In such a system, you could probably swap in papers with writing on them instead of blank ones, and bind them that way. It also implies that you'd be able to destroy the book to remove the papers... but then we'd need support for writing on both sides of the paper, and what about title pages? So that's a mess.

In the short term it would probably be easier to just let you read a piece of paper from inside a folder without taking it out. That would be easy.

Striking through previously written lines: It's feasible to implement, but writing isn't grouped internally based on lines, it's grouped based on paragraphs. The easiest implementation would be to let you cross out entire paragraphs. That said, arbitrarily crossing out portions of what you've previously written (or even scratching it over entirely to make it illegible, as some people currently roleplay doing when reusing paper) is on the table as a potential feature.

Table formatting in books: Godot already has table/cell BBcode tags that should be enabled for writing. We're not going to document them, but if you want to use them, go ahead.

Preview functionality: Godot currently (or did? I haven't checked recently and they update quite often) has an issue where text fields have a thin margin on the right side that's impossible to remove, so even without any tags at all, what you've written won't match 1:1 what shows up on the paper since the lines might break differently. I'd want to resolve that before I even thought about BBcode. And, again, BBcode is more of a "bonus feature" for power users than something we officially support or encourage people to use. It's in the game because it was already implemented in the underlying engine and didn't break immersion, so it's available if you want to use it. There will never be a BBcode toolbar in the writing UI.