From Bulbapedia, the community-driven Pokémon encyclopedia.

It is certain that there will be remakes of Pokémon Gold and Silver for the DS, and I think they will be called Pokémon Promethium and Neptunium. The reason that I think they will be Pokémon Promethium and Neptunium is because Promethium is named after Prometheus who, in Greek legend, gave fire to people, which fits with Ho-Oh, and Neptunium is named after Neptune, the Roman god of the sea, which fits with Lugia.

You will be able to visit all four regions, and have the National Dex from the start, but you won't be able to visit Hoenn or Sinnoh until you have defeated the Kanto/Johto Elite Four, which will now require you to get all of the badges from both Kanto and Johto to be able to challenge it. Once the Kanto/Johto, Hoenn, and Sinnoh Elite Fours have all been defeated, you can challenge the Ultimate Elite Four, which will have Pokémon from level 75 to level 100. You still have to get the badges to get other trainers' Pokémon to obey you past a certain level, but Pokémon from Hoenn or Sinnoh that are another trainer's won't obey you if you haven't been to that region yet and the Pokémon is over half the level that your badges would allow.

All of the legendaries will be catchable, including the event legendaries. The areas where the event legendaries are will be accessible without the event item, except for Arceus, which you will still need the Azure Flute to catch, but you will be able to get the Azure Flute by defeating the Ultimate Elite Four.