@Tamiel:
Eeee thousand of copies sold on the Joker's backstory is a thing. I think it is called The killing Joke, okay I lied, I know it is called that it even has a movie. http://www.comicbookresources.com/article/batman-the-killing-joke-tops-bookstore-sales-28-years-after-original-release
I've seen the movie (kind of a "meh" movie, not particularly good, but not that bad either), and Red Hood is not necessarily Joker's backstory. He creates different backstories all the time.
–- Update From New Post Merge ---
@Count:
I respectfully disagree. There are way too many major mysteries to ignore when it comes to him, and having elaboration would only make his character more endearing to know what his endgame goal is rather than just doing everything he does for the lol's.
Does he really have a goal other than "live freely, destroy everything, become king of the pirates"? What made me wonder about this is his personality and his multiple allusions to the concept of "destiny" and "luck". Seems like the only thing that truly guides him is "destiny".
I think that out of the "alignment chart" categories, this is the description that matches Black Beard the most:
!
"Some men aren't looking for anything logical."
Not that I think Black Beard is the One Piece version of the Joker, by the way.
So does he really have a true, precise goal that would make him more interesting and would need to be developed in the future? If not, then a flashback that would explain his goals won't happen. Following this logic, we may get a flashback in which we are told why Black Beard is crazy and acts in such an unpredictable way, though.
Again, and just like the Joker, I wouldn't like to see this. Real chaotic, psychotic characters don't need a reason! (that's how I perceive things)
@Count:
Luffy, who is similar to Teach in personality, ambition, and mysteriousness, didn't need his flashbacks post-Marineford or his major lineage reveals with Garp, Dragon, and sort of Ace/Sabo to be a simple likable character as the protagonist. And we always barely knew anything about him and there are arguably still things we don't know that can still be revealed. But showing them made his characteristics and growth a lot more endearing, along with feeling a lot more intricately connected with the One Piece world.
Even though I feel Luffy's flashback was made to show his relations with Ace (and thus insist on the pain of losing him) and to introduce Sabo and the rotten lofty environment he lived in more than to develop Luffy's character, I understand what you mean.
What I disagree on is the belief that a flashback would make the character more endearing. It could, in fact, if badly handled, make him less interesting and less realistic in the story he's evolving in, as well as to us readers.
Besides, Black beard doesn't need to be appealing; he's a bad guy. He captured Ace, betrayed White Beard, he even tried capturing Luffy.
He should stay as he is; someone we can't have empathy towards yet that we find intriguing, special. What I like most about him is the mysterious fog surrounding him.
A sad flashback that would justify his evil actions would, I think, ruin it all. Especially if it's pure cliché like "Teach was bullied at a young age. Now he wants to destroy everything and everyone."
@Count:
Blackbeard's made too many major plays with long-term planning that involves literally YEARS of buying time and putting up acts to fool both a Yonko and the World Government to just leave it ambiguous as to what his goals are or just doing it for something as simple as being bored. Not to mention how he knows the whole Devil Fruit stealing trick and has an atypical body that allows him to store two, along with scarring the Yonko and somehow getting away with it despite having no reputation or bounty. Leaving such a major force that seems to defy all of the established preconceptions of this universe unchecked is more likely to harm the story as seemingly lazy writing with no elaboration than benefit it as a cool mysterious factor.
Things like "how does he have two devil fruits?" and "why is his body described as special?" need to be explained, yes, and they surely will.
But his plan to fool White Beard was only due to him wanting Tatch's devil fruit, and fooling the World Government to get a powerful crew and then steal another powerful devil fruit; basically, to gain power. His quest thus far was a quest for power. I think he needs this power to dominate the seas and be (his own definition of a) pirate king.
I'm surprised the World Government isn't taking his case more seriously - he has the the power and the temper to literally destroy the whole world.