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While I get where you're coming from with the possible cooldown, lore-wise Aegon conquered the seven kingdoms (six, I guess, not Dorn) in a remarkably short period of time. Adding a 5yr cooldown would make that conquest take at least 20 years, so it simply wouldn't work in key scenarios like that. If there were some way to limit it to continents, that might help.

And while dragon conquest is over powered, you have to remember that unlike an invasion CB everyone you're fighting retains their titles. So succession will be a nightmare, as a whole bunch of kingdoms are going to rebel.
 
Dragon games are fine as long as you use self-control. Don't just invade everybody, only launch realistic and character motivated attacks. For example, if you start as Aegon and create New Valyria then don't conquer anything past the Free Cities in his lifetime because in roleplay terms it would take 40 years to consolidate those gains, even if the mechanics don't really reflect that. Once you get a new ruler who is similarly war-adept and ambitious, then you could invade the Vale or Slaver's Bay. A strong administrative ruler could also try and consolidate within your borders by integrating the Andals and the Rhonyar, and rebuilding the ruined cities. Keep going like this and you should be nowhere near conquering the world before your dynasty starts crumbling from within and things get really interesting, with dragon wars and divisions of territory. I also like to spice things up by using the console to model situations too complex for the engine, like two brothers fighting for the throne of a huge empire and reaching a stalemate so you divide the empire in half, or a Dothraki invasion capturing the disputed lands but leaving Tyrosh and Lys unharmed because of their island status. Once you get your mind around the idea that the game can be about modelling alternate history scenarios rather than being a challenge, the sky's the limit.