Inspired by Hear Me Roar:
Stannis in the War of the Ring.
One week to the day before the Battle of the Blackwater-Stannis is ISOTed alongside his fleet and two armies to Belagear. He finds with him, all the islands under his authority in the narrow sea-Driftmark and Dragonstone, with his mercenaries from Myr alongside all his forces south of the Blackwater Rush. As well as the garrison in Storm's End.
Dragonstone is about 20 miles off the coast from
Lond Daer - Tolkien Gateway. Alongside Stannis's fleet and other resources. About 20 miles southwest from the port.
Lond Daer Enedh (also spelt Ened) was a great harbour in Eriador founded by the Númenóreans.
tolkiengateway.net
This is within sailing distance of both the Grey Havens and Gondor. But is far enough from both that contact will not be immediate.
Stannis is ISOTed around 3015 T.A. About four years before the War of the Ring.
This will give him enough time to scout the area and also maybe allow some of the excess population to settle the abandoned port.
Though contact with Gondor shouldn't take too long as well as the Elves.
Stannis's bid for the Iron Throne is sunk, but now he is in a war against a much greater menace than the Lannisters and all other usurpers.
I expect some of the mercenaries to be lured away with promises of loot and wealth from Sauron. Also Melisandre will need to be watched carefully lest she succumb to the dark lord's will when scrying him through her flames.
Though Stannis siding with the good guys-should be fairly assumed. As he has no claim to any land here(beyond what will be given to him) and he isn't in the business of conquering.
You could have some tension between Davos and those who want to join with Gondor and possibly the Queen's Men if not Melisandre who see Sauron as R'hllor embodied. Or those who want to simply carve out a kingdom in an abandoned and depopulated land and let the two sides fight it out.
Anyway Stannis iron will vs the ring of power(or Sauron via palantiri) would be a true test of his mettle.