Sins has been a great success for IC, I doubt they would drop the franchise. From the past devs communications, the 'micro' expansions were not costing micro-resources to make, therefore Diplomacy should be the last.
From that, I think one of 2 options are possible:
They will try their luck on a brand-new franchise, and I would be very curious on what could possibly come out, given the quality of Sins; The devs can be a bit tired of the same theme and may want a breath of fresh air, and it will be even better if it is at least as successful as Sins.
They will gear up on a Sins2 design, probably with a brand-new engine which gets rid of many of the (expansion development, modding) limitations of the current one, to make room for a great reimagining of the gameplay, with future expansions in mind from day 1. Which I would like to see too.
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Yeah, i would like to hear from an official mouth which way IC is going in the future...