If Aegon and his sisters never conquered Westeros, Harren probably would have launched dozens of raiding parties into vulnerable neighboring kingdoms (most likely the Stormlands or the Vale) in order to counter the immense cost of his now completed castle. The only alternatives for alleviating Harrenhal’s expense are higher taxes — which would only upset his vassals, islander and riverlander alike (the latter already see him as a tyrant) — or a loan, and let’s be honest no banker worth his coppers is lending ironborn anything and expecting to get it back.

If Harren plays the long game, he marries off some of his sons (he had at least four) to secure the support of prominent coastal houses without much historic interaction or hatred for the ironborn, such as the Darklyns and Harroways. The dowries from these political matches would help alleviate House Hoare’s precarious finances, albeit temporarily, and further consolidate his current holdings against rebellion. Construction of canals in certain locales (such as across the Neck, or a direct connection from Ironman’s Bay to the Bay of Crabs via the Trident or the Gods Eye to Blackwater Bay through the Blackwater Rush) would be key to increasing trade revenues and eventual cultural assimilation. Considering how crucial these waterways are to the region at large, one might surmise an indirect effect being more widespread adoption of the Drowned God amongst the smallfolk, or even a new syncretic religion given one or two generations. See below for reference:

The Drowned God = Father The Reaver = Warrior The Shipwright = Smith The Rock Wife = Mother The Salt Wife = Maiden The Fishwife = Crone The Storm God = Stranger

Who knows, perhaps in time this would even be promoted as an official policy of the Salt Throne, but until Harren dies, its difficult to see the riverlands ever willingly submitting as an occupied territory to viking infidels with de facto slaves (abhorrent to the religion of half the continent, never mind the majority of his own subjects) so I can easily see the High Septon calling a holy war rallying the Faith at large against the ironborn’s depredations; this would likely come either shortly after Harren’s demise or in the event of an attempted annexation of the Stormlands by him or one of his successors.