OK THEN.
The game hasn't been leaked, but we have a heap of general info just released already, on serebii.net.
Please note that this has not been 100% confirmed, but it certainly looks like this info can be trusted - would be surprising to see it turn out to be false.
So far, we know the following:
Legendary Pokemon:
- Lugia and Ho-oh both obtainable in HG/SS in the same way they had been in GSC - one via the Silver/Rainbow wing given to you when you beat Team Rocket, and the other is given to you by an NPC in Pewter City.
- Suicune can be caught when you encounter it in the overworld by Bill's House in Kanto.
- Articuno can be found in Seafoam Islands (meaning Seafoam Islands has been expanded, and hence probably other places unlike in GSC), Moltres in Silver Cave, and Zapdos outside the Power Plant, each at level 60.
- Groudon, Kyorge and Rayquazza can be found in a cave west of Cianwood. They are activated by obtaining a Blue/Red orb from a man on Route 27 after getting the National Dex, depending on your version (SS = Groudon, HG = Kyorge).
- Latios/Latias in the game, roaming in Kanto at level 50.
- Entei/Raikou roam Johto at level 40.
- Mewtwo can be found in Cerulean Cave at level 70.
More detail here:
https://www.serebii.net/heartgoldsoulsilver/legends.shtml
Starter Pokemon
- On top of choosing one of the three Johto Starters, you can choose one of the Kanto starters upon getting all 16 gym badges...
- ...and your choice from the Hoenn starters from an NPC in Sliph Co. upon beating Red at Mt Silver. Both sets are at level 5.
More info here:
https://www.serebii.net/heartgoldsoulsilver/starters.shtml
Various other tidbits
- First, Pal Park is in this game and is located in Fuchsia City where the Safari Zone was in FireRed & LeafGreen.
- Next, you can trade from Diamond, Pearl & Platinum immediately after you get the Pokedex.
- Snorlax is findable outside Diglett's Cave again at Level 50.
- Red is findable in Silver Cave again, with a slightly different Pokémon Roster
- It appears that the Odd Egg is back.
- It appears from pictures that there is a Rocket Admin, or at least a higher-up who you fight early on (before Bugsy, maybe in/before Slowpoke Well).
- Shiny Pokemon may follow you while appearing as their shiny forms in the overworld. (Not quite confirmed yet, does apply to Red Gyarados though).
- You team up with Lance and battle with him in Team Rocket''s Hideout in Mahogany Town.
- You get an Apricorn case from the guy in the house directly north of Cherrygrove - only then can you pick Apricorns. (One by his house is green).
- Trading is possible immediately after you get the Pokédex. No restrictions
- Celebi is #257 in the Johto Dex
- Battle Recorder received from Hibiki/Kotone in Violet City
- If you're in a small area and have a big Pokémon follow you, it will return to its ball
- Arceus will get you to Shinto Ruins(teleportation) even if you've just started the game, though you need to box the rest of your Pokemon to get Giratina/Dialga/Palkia at level 1.
- On top of a Team Rocket Executive called Lance (NOT the Champion Lance), the is another male executive called Lambda and a female Executiver who appears in aforementioned double battle against you and Lance, called Athena. All the Rocket Executives are named after different types of real-life Rockets.
Also, some (unconfirmed but likely to be real) pictures from (a lucky) someone who seems to have gotten the game early (these are large). PLEASE CHECK THE THIRD POST FOR EVERY PICTURE FOUND THUS FAR.
First twenty below:
2nd in particular shows that Lapras is supposedly back in the same place it had been in GSC.
Gym stuff
Reportedly, Falkner has a Level 9
Pidgey and a Level 13 Pidgeotto. And gives out TM 51. (Roost).
Bugsy Has a level 15 Metapod and Kakuna (imagined to have Bug Bite), and a level 17 Scyther with a Sitrus berry. He gives out the TM U-Turn, apparently. His Pokemon are each pne level higher than what they had been in GSC.
So far, it appears the original rosters are kept, but with new movesets and levels.
Whitney has a level 17 Clefairy, and a level 19 Miltank with Lum Berry. The TM given is Attract. This is below what it was in GSC by 1 level for each Pokemon... unknown if there's a reason behind it or not (Miltank having Thick Fat ability, or less trainers between Ilex Forest-Goldenrod than other routes, lack of expectation/ability for people to train north of Goldenrod for instance, etc).
Morty has a Level 21 Gastly and Haunter, a level 23 Haunter, and a level 25 Gengar - same as GSC. He gives the TM Shadow Ball.
Chuck has a Primeape at level Lv. 29, and a Poliwrath at Lv. 31. He gives the TM Focus Punch. Higher than GSC.
Jasmine has two Magnemite at level 30, and a Steelix at level 35. She gives out the TM Iron Tail.
Pryce has a level 30 Seel, 34 Piloswine, and level 32 Dewgong. He gives the TM Hail.
Clair has team of 3 level 38 Dragonairs and a level 41 Kingdra.
Sabrina has a Lv.53 Espeon, Lv.53 Mr. Mime, and Lv.55 Alazakam.
Janine has a Lv.47 Crobat, Lv.44 Weezing, Lv.47 Ariados, another Lv.47 Ariados, and Lv.50 Venomoth.
Gary's team - Lv.55 Exeggutor, Lv.58 Arcanine, Lv.58 Rhydon, Lv.52 Gyarados, Lv.56 Machamp, and Lv.60 Pidgeot
Red's final team - Lv.88 Pikachu, Lv.80 Lapras, Lv.82 Snorlax, Lv.84 Charizard, Lv.84 Venusaur, Lv.84 Blastoise.
(Last two found from hacking, apparently).
Pokemon Location stuff
Sentret, Zubat, Weedle, Kakuna have been confirmed to be found before Violet City. (No surprises there).
Vulpix is found on Route 36.
Mareep is on Route 32, 42 and 43.
