Since most of High Rock's cities are coastal, we could see a pronounced naval aspect in TES VI, should it be set in High Rock. From the Nordic raiders and traders of the Sea of Ghosts, to the corsairs of the Iliac Bay, ancient underwater Dreugh cities and Ithguleoir, the immortal leviathan that stalks the Eltheric Ocean. We are bound to some some references to the Cthulhu mythos, as seen in previous games. A sailing mechanic would be interesting, considering the various islands of High Rock, from Betony and the Dellese Isles in the south to Caecilly and various small islands and inhabited icebergs to the north.
And when it comes to DLC, High Rock also offers a lot. The Bangkorai Garrison essentially functions as a passageway into northern Hammerfell, which could allow us to explore the area featured in Shadowkey, namely Dragonstar and its few surrounding settlements. Sentinel and its surroundings could also certainly work as DLC, being isolated by the Alik'r Desert, and since Sentinel is officially ruled by the Lhotunics, it could function as a microcosm of the Forebear-Crown conflicts. Finally, there is the Battlespire. According to "The Doors of Oblivion", the Weir Gate's link to Oblivion had been broken.
What if the link was somehow restored? The Battlespire may be a shattered ruin, but should the forces of Oblivion lay claim to it once more, this could have devastating results for the denizens of Tamriel. The Battlespire itself would be a massive, multi-leveled dungeon, connecting to various Daedric Realms. The main quest could revolve around various factions from both Tamriel and Oblivion, trying to lay claim to the Battespire, from factions like the Synod and the College of Whispers, to groups of Daedra and the Daedric Princes themselves. The overarching threat would be that of another Oblivion Crisis.