There are other things on BB that bother me too. His Yami Yami no Mi is pretty damn powerfull but very risky as well, because he isnt invincible (except haki) like other logia fruits - But still(!) he can take looots of damage and he always stay up again. Its terrifiing! Just like Luffy - i like to refer at this point to the fight of luffy agains Don Krieg, Crocodile or Lucci. Luffy got down so often, but he has a willpower out of this world which made him stand up until the fight as over.
I guess this kind of willpower has something to do with the "D" - the willpower to achieve the goals, or die trying.
How hard would it be to see Luddy and BB fighting each other. No one would give up so easily... I can even imagine that this fight will go to either ones death.
But thats too far fetched now.
So what i wanted to imply is that it might could have something to do with the "D" - i for myself think the Yami Yami no Mi theory, that he absorbed the Power and keeps it because of his Yami Yami no Mi, is way more logical. But the Willpower to survive and reach the goals no matter what could be an indicator why it is only accessable for the "D" successors.
Now it gets across threads but i want to add something interesting here:
If it really is because of the "D" - and the fact that the People with the "D" are very special - that thats the reason why BB can have 2 DFs, then i think that Luffy himself could get the Mera Mera no Mi.
As a counterpart of BB he may have to get a second ability.
We also know that Oda gives us unclear predictions and foreshadowing from time to time. And the Jump Cover from the issue where OP Chapter 699 was in bothers me the most of it:
I know that he already has his Red Hawk technique which implies heat into his attack so it might just be a refeerence to that.
But well, i give the Theory of People with "D" being able to eat 2 Fruits barely 20-25%
If thats True i give Luffy a chance of 70-80% of getting the Mera Mera no Mi for himself.
The Theory that BB absorbed WBs ability just because of his own ability, like he can absorb and hold any power as long as he wants to a 70-80% chance as well.
So you see, the Theory i was talking about i found unlikely, but interesting enough to tell you guys to bring you to new ideas for this topic. (And for the Mera Mera no Mi topic)