The Territory Tax
The designers promise "the game will never offer players a paid-for advantage over opponents – the only way to build a successful empire will be through commitment and planning."
This is nice, but perhaps we could go one further: not only will players not be able to buy advantage over other players, but players that do amass large amounts of territory (above a certain threshold of ridiculousness) will need in-app purchases in order to retain their empires at baseline levels.
Given that this game could require significant ongoing server infrastructure and attendant costs, this could be a way of keeping the whole thing sustainable.