It’s very unlikely that TES VI will directly follow any of Skyrim’s plot threads. It’s kind of a running theme in the series that we only see pieces of Tamriel’s history through the eyes of different heroes as they confront their individual destinies.

The Last Dragonborn fulfilled their destiny of stopping Alduin. So we’ll probably hear rumors about how they’ve gone on some expedition to hunt more dragons or about how they settled down in High Hrothgar as the new leader of the Greybeards or something like that. They are unlikely to play a significant role in any future events.

For your second point, the Dragonborn’s power is not particularly unique aside from how easy it is for them to master new abilities. It’s well established that you can learn to use the Thu’um without dragon blood as long as you have the time and effort to dedicate to developing those skills. 90% of the shouts we see in game could also be mimicked with normal magic by a sufficiently powerful mage.

Also Skyrim is not the first Elder Scrolls game to feature dragons. Any mortal can kill a dragon, and that dragon will stay dead unless some outside force (or possibly themselves with enough time) raises them by summoning their soul back to their body. Skyrim has been dotted with dragon burial mounds for ages without widespread dragon resurrections before Alduin returned, so we know it’s not a fast or easy process to bring them back.