OK, so this is something I have always been curious about. So we know that the War of the Ninepenny Kings took place a couple decades before the War of the Five Kings. It started when a band of 9 not-kings got together and agreed to cooperate and press their claims to various kingdoms. While we know who two of the kings were, the rest remain a mystery. These are my thoughts:
So the two kings we know about are as follows:
Alequo Adarys - He convinced the others to take over Tyrosh and put him in charge. It seems like he was the leader of the group.
Maelys Blackfyre - He was the last of the Blackfyre pretenders and commanded the Golden Company. He planned to conquer the Seven Kingdoms.
Now seeing as they first took Tyrosh, I would guess that two of the others ruled Myr and Lys, giving them the ability to conquer the Disputed Lands so easily.
As for the rest, it really depends on how large the war was. If it was just a small regional conflict then the other five were probably just a bunch of pirates with claims to parts of the Stepstones. However, if this was a larger, global conflict then there are some interesting possibilities. This is my theory of who the other five could have been:
The Sealord of Braavos - He would have provided the funding for the operation. He would be removed by a coup after failing to pay back the Iron Bank.
The Prince of the Summer Isles - He would have joined the Band of Nine with a force of exiled Summer Islanders. His expedition would fail, leaving Jalabhar Xho on the summer throne.
A Merchant from Pentos - This would be the sort of Illyrio of the operation. He would be seeking control of Pentos and would bring funding and sell swords to the cause.
The King of Astaphor - He would be operating mostly on his own trying to conquer Slaver's Bay, but he would provide some Unsullied to the war effort.
The Praetor of New Ghis - He would be trying to conquer New Ghis with a legion or two. His disciplined troops would be vital to the band's war efforts, but he would rely heavily on the Sealord and Merchant to pay his troops. They would desert and kill him when the money ran out.
Anyways, those are my thoughts. What do you guys think?