Just an idea I thought I would jump in with...
Why not make the Inca very prone to rebellions? Perhaps have a chance that, every time the faction leader dies, a 'Rebel Inca' faction will spring up?
From what I've read, power struggles were pretty common after the death of a Sapa Inca, and some of the resulting wars could be quite bloody. Apparently the Spanish arrived after a particularly nasty war.
Such a scheme would seriously limit the Inca ability to expand, since they would need to keep sizable armies around to deal with rebels, hence limiting their cash intake, while allowing them to keep their historical battlefield l33tness.
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