I know it's not good to think like this, but once I heard that he made Supernova on the spot, I automatically think that many of these things are done on impulse and he builds from there. I remember reading some comments about him being surprised at how much involved Law is. I'll have to find the exact quote. It might've been translated by Sandman from a OP Log either 26 or 27.
I suspect that for many entities currently, but can be wrong and they were mostly in his plot notebooks. It does feel like Morgans was planned a while ago tho. The person who is in charge of the News Coo doesn't seem like an on the fly. The Underworld in general doesn't seem like it stems from impulsive creativity.
What is funny is that when I bring up the Research team or Underworld leaders, most seem like they don't care because none of them seem big and powerful. I find it interesting because information, funds and resources in general have been shown to be crucial in the OP world. These individuals, along with others not shown, are considered big shots in that field. Also, the fact that some were noted by Jinbei as big deals to be mindful of
He definitely creates some stuff up on the spot, especially with a series running this long, and the things he likes he continues to expand on, I think I remember reading that same interview you mentioned about law, who I honestly love (I know a lot of people on the fourm aren't of the same opinion
and one thing Doflamingo's down fall showed us, is that Brute strength isn't the only type of power that can change the world, his criminal operations and then lack thereof upon his defeat completely changed the balance of power, so yeah I'm very interested, like you, to see who else these mysterious puppeteers are, power levels be damned