I would love to see a true binary-planet... Where two relatively equal planets are sharing the same center of gravity.
ok... Maybe one might technically be the moon because it is only 90% of the size of it's peer.
But still, that would be interesting because we could see if the two planets were exactly the same chemically: Same density. Same Atmosphere. Same biological markers?
If most planet formation theories are to proven out, then they should be all but identical twins...(Even if they wern't in the habitable-zone it would still be incredibly interesting)
Here's to hoping we see a discovery such as that near-by the Earth and in the near future...