I’m skeptical of any negotiations with the ironborn in terms of marriage alliances or political alliances, such as the popular idea of Robb marrying Asha if he keeps his hand open. They seem so culturally other (especially in POV chapters from the Greyjoys) that it seems they would be incredibly fickle. The best scenario would be something akin to the Red Kraken where his nominal allies just let him attack their enemies and hope for the best without any higher strategic considerations.