I would have to place my bets on:
Lugia and Ho-Oh will be encountered on their respective versions at lv.47 (like Dialga, Palkia and Giratina in their respective versions), while in the opposite versions (e.g. Ho-Oh in SoulSilver) will be lv.70, just as Dialga and Palkia were in Platinum and Groudon and Kyogre were in Emerald, also weren't they lv.70 in the original Gold and Silver?
Legendary Beasts - I would have to say they will roam at around level 50, as all runners post-Lati@s have, it would also fit better into the game if they did choose to alter the levels of opponent's pokemon to make it harder.
As for additional legendaries, Celebi will 99% be in the game as an event. I would expect to encounter it at around level 30, as with the Faraway Island Mew and Flower Paradise Shaymin of past games.
As for other legendaries being in the game, beyond those it goes into speculation. All the trios will be available in the fourth generation following this game, so I doubt we'd see any of those. The white rock in Kanto thing raises questions, as they did re-use events in Firered and Leafgreen for Emerald, so we may see Shaymin again. Also, I'm a bit confused as to why they've announced this game, but no Arceus WiFi event for Platinum, which makes me think that either they will delay the GS Ball event (which considering how close the Secret Key event was to Platinum's release seems unlikely) that Arceus might also be available in this game, which would actually bring the total up to three, putting it the same as DPt and one behind Emerald (I see no reason to add in Darkrai really). However, Arceus may be a bit of a dodgy move, as it is linked deeply into Sinnoh's mythology, which is why I don't expect to see Groudon, Kyogre or Rayquaza in these games.