Fascinating question. Well, if we assume that Valyria had continued in it's path, several powers in Slaver's Bay would not have risen in dominance and the power would mainly be kept there in the Freehold.

The real interesting question is Westeros. Without Aegon the Conqueror, I highly doubt that any one king could have ever united Westeros. The realm was just too politically divisive. So most likely House Hoare, House Gardener, and House Dundarrion would still be in power and House Stark, Lannister and Arryn would still be Kings.