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I think that depends on wehre the Diamond/Pearl region is. It would be a little weird if it's far away from the other countries, and the characters are siblings....but if they're just related, that would work....^^;;
Another new thing they could add....older trainers. Not only two or three years, but maybe twenty or so. (And I don't mean the gym leaders or the elites or other already known persons.) Though I don't know if that actually happened before...
A brand new character would be really great...if D/P have three starters like the other games, there may be a third character....or they will wait for a third game to come.