Yes, we know they are leaving the Vampire Diaries, to star in a show 

Called the Originals.

Niklaus, Elijah and Rebekah are the lead characters on the show.

there not leaving the show per say , they are starring in a show that doesnt mean they wont be in the vampire diaries though. Pretty sure this show about them is going to be about there family like 1000 years ago and how it will lead up to the events of the vampire diaries etc.

I dont think Originals would be a show going too far back , like sure it will have flashbacks but it will be set in the present time and will be concurrent with TVD so that at all times cross overs are possible and events can overlap like by the narrative maybe if there is say a big problem in mystic falls we hear quips or passing mention of it in the Originals

I think it's a fantastic idea. It's almost here! There are so many possibilities for story lines here for a family that has lived a 1000 years and has been all over the world. And think of the overlapping story lines! The Originals were too big for little Mystic Falls. The Originals will still be around (if the show takes off then the set will move to Atlanta right next to the TVD set).

Please check out the Originals Wiki! and help better it please