Technically, Zelda does reuse its own content and characters. Link, Zelda, Hyrule, and all the different races are reused. Just because a game reuses characters does not mean it has to be boring. Also nobody ever said that Dark Link just has to be a mirror of everything Link did or does.
He could have his own personality and his own skills that are completely different from Link. A story about his ressurection in from being slained in the Water Temple in OOT would be a pretty good start.
The power of the dark side is strong, but America is stronger.
God bless the father land.