My theory is that the national treasure that doflamingo was referring to is one of the ancient weapons Uranus! Uranus I believe is the devil fruit tree. The celestrial dragons don't eat them because the devil of the fruit would taint their "purity" So they make bargains with the underworld. At the same time Not telling anyone about uranus. The gorosei at the took the most powerful devil fruit. each of them ate a part of it. Reason why they have lived for so long. Doflamingo got ace's devil fruit from the celestial dragons.
Also something I want to add. I believe blackbeard did not eat 2 devil fruits. More specifically He sucked the devil out of whitebeard and implanted it within himself. I believe his Yami Yami no mi allows him to transform into different hellish creatures. (3 headed dog, minotar, etc.. A reason for Shanks' scars on his eye.
Not a bad theory, but it contradicts a couple of things explicitly stated. Firstly, we've seen a DF return to circulation. When Smiley died on Punk Hazard, we saw an apple in a sack of fruit turn into the new Sara Sara no Mi, Model: Axolotl. So unless Uranus is a gigantic tree that bears enough fruit to compete with the rest of the world, I don't see it being likely. It's definitely feasible, but they definitely don't have a monopoly on DFs.
Secondly, back in Enies Lobby when Spandam gave Kaku and Kalifa their fruits, Jabra had a whole panic attack about just being close to a second DF, only for Blueno to call him out on that.
Unless there ends up being a plot twist where it turns out there are literal Devils inside of every fruit, I'd expect that Oda just doesn't explain WHY devil fruits work. I'm fine with explaining the rules of DFs, sure, like how they can return to circulation and availability, but I personally think it'd just be better to say "They just do. Don't question it" than to focus on having demons inside them and all that crap. That's kinda what happened at the very end of Naruto, because it felt like it was focusing too much on the origin of the Rinnegan and Sharingan and Byakugan more than the characters.
For the record, I'm not against knowing how they work, I'm against derailing the focus of the story. One Piece's focus was never on how DFs work, it was about Luffy and the journey of the Straw Hat pirates. It's never been a major plot point until Blackbeard showed up and stole the Gura Gura, but I like that while we debate it, the actual story never questioned it or delved into it thus far, and I think because Luffy almost certainly doesn't really care how the fruit works, it won't affect anything too much. Otherwise it could end up turning into Luffy's Gomu Gomu demon fighting against BB's Yami Yami demon, and at that point it's basically just Naruto. Or Toriko. Take your pick.
