First of all, I'd just continue Stark's descent into villainy. After everything the man has done and suffered, there's no conceivable way that he'd still be a "super-hero". So the gist of it is that he becomes disillusioned with the "hero" game, and fucks off,creating a mobile platform/base in International Waters. He decides to invest more time in his Corporations, and actively tries to absorb and destroy the ones that stand in his way, in order to build Stark Universal. At the end he upgrades the Base enough to create a whole City, and starts populating it with people he thinks are worth it, based on cold, hard analytics and logic. So something like the Maker, but without turning people into drones. More similar to Ryan's Rapture.
Either way, now he's basically heading a brand new country, while being head of one of, if not the, most powerful companies in the world, and his mind is focused on expansion. So he tries to work within the system, destabilizing certain countries, forming alliances with others, all moving towards his goal of a perfect world.
TL;DR He turns into a mix of Big Boss & The Illusive Man.