As the game is close by, Serebii.net, 2ch.net has updated lots of news.

1) Gym Leaders have new Pokémon.
2) Legendary Pokémon,
http://www.serebii.net/heartgoldsoulsilver/legends.shtml
3) Starter Pokémon
-Johto's: Start of game
-Kanto's: 16 badges
-Hoenn's: Defeating Red and meet man in Silph Co.
4) Miscellaneous
a) Pal Park is in Fuchsia City (over Safari Zone)
b) Trade from Diamond, Pearl & Platinum after getting Pokédex.
c) Snorlax is findable outside Diglett's Cave again at Level 50.
d) Red is findable in Silver Cave again, with different Pokémon
5) Shiny Pokémon following you, will be shiny when outside their PokéBall.
6) Team Rocket
a) A new Rocket Executive in Mahogany Hideout, called Lambda.
b) A female Executiver is also there in a double battle, teamed with Lance against her, called Athena.
c) In Goldenrod, you battle all the executives again and then a final executive, Apollo and he has a Houndour (Lv. 35), Koffing (Lv. 35) & Houndoom (Lv. 38).