Well as everyone probably knows now, HG and SS will be following in the footsteps of FR and LG by having a crap load of legenderies in there. So far not only do we have Lugia and Ho-Oh but also Groudon if you get SS, Kyogre if you get HG & Rayquaza.

They are also continuing the tradition of Platnium by having Level 1 Dialga, Palkia or Giratina in the game if you bring in the movie Arceus. As well as unlocking a secret location with Cynthia there.

ALSO if you bring in a Jirachi it will ALSO unlock a secret location

All thats really left is Mew and Mewtwo coming in!