I'd love to see a whole new zone rise from the sea and the associated mystery of who built it and what happened to their civilisation form the bedrock of a chapter (I absolutely can't stand planes of oblivion and daedric prince of the week, but this seems a more grounded way of achieving a similar break with the nearly-full Tamriel map).

Probably a lore breaker although there's no reason why these things wouldn't have been forgotten with the later games if, for example, this zone sank back beneath the waves some time hence.