Coolest niche empire?
I'd say it's the Venetian Empire. It was essentially a banking city-state that relied on mercenaries and wealth to fuel its expansion. Every new territory established more trading posts, generated even more banking revenue, and strengthened the tiny city's monopoly over trade.
Venice's dominance—along with the Ottomans' control of trade in the eastern Mediterranean—eventually pushed the Western European powers to finance voyages in search of new routes to the East, leading to the discovery of the New World.
I just find it fascinating how much power such a small city had.