I personally think that the Summerset Isles would be an optimal last entry in the current story arc revolving around the rise of the Thalmor and the further decline of the Empire. The Altmer's culture has been shaped by a history of conflict with outsiders and a rigid sense of traditionalism, forever bogged down in mindless ritual. Selective breeding between the bloodlines to maintain a sense of purity. And of course, infighting. Auridon, the shield of Alinor, has suffered many wars between the kinholds of Firsthold and Skywatch. It is no different in Summerset, where the historic rivals of Lillandril and Alinor have clashed many times through history. The south, encompassing Sunhold, Dusk and Isque, is shaped by continuous conflict with the Maormer of Pyandonea, only ceasing after the loss of Orgnum's Coffer in the War of the Isle.
So far, Alduin has been the main antagonist of the first installment of the new story arc. We've had the Time God, what else is there? Well, Hammerfell or Elsweyr could provide an antagonist in the form of the Missing God, Valenwood could provide an antagonist in the form of the Earth Bones, and again, Alinor or Elsweyr could provide an antagonist in the form of the Brass God. After a catastrophic loss of power, perhaps due to some crisis in Valenwood or a failed invasion of Hammerfell, the Thalmor would be weakened, perhaps weak enough for factions in Alinor opposing the Thalmor, perhaps traditionalists that view the Thalmor as upstarts, corrupting the purity of the Altmer and their culture, or radical reformists, or the Psijic Order, could take a shot at overthrowing the Thalmor and restoring the kinhold system, or gaining dominance.
Certain situation and crises could also result in other non-mer races being allowed in the Summerset Isles. Mercenaries, merchants, diplomats, agents and allies of the Thalmor, and of course, servants or slaves. A failed Tamrielic invasion of the Summerset Isles could result in this for example. A massive fleet, headed up by Redguard anc Colovian galleons, burned north of the Cape of the Blue Divide by a massive flying superweapon of the Thalmor - a Sunbird of Alinor. The player character - a survivor, is captured, but set free by the Beautiful, a terrorist organisation working against the Thalmor.