- Regigigas will almost definitely be in Hoenn, considering it's the home of the Regi Trio and some of the original Braille text even mentions him. He'll probably be slumbering in a hidden room of the Sealed Chamber that only appears after you get the National Dex. You'll probably need to have all 3 Regis in your party to awaken him.
- Either the Beast Trio, or the Genie Trio might make an appearance, the former being fan favorites and the latter making sense with the whole weather thing (also Landorus representing fertility in the region that introduced Berry growing). You'll probably be able to encounter only one of the three depending on which starter you picked.
- Heatran is also a possibility, because Hoenn has a lot of caves that he could be hiding in. He could even end up being in Mt. Chimney, if they don't end up bringing Team Magma's base from Emerald back.
- I would love for the Creation Trio to make it into the game, because - having skipped Gen IV - I never had a chance to catch any of them. But, I don't see how they would fit them in Hoenn, without making cop-out, box-shaped areas like "Time Room", "Space Room" and "Antimatter Room".
- As for event-exclusives, Jirachi and Deoxys are the most likely candidates. Deoxys' form-changing meteor could be located in the Mossdeep Space Center. Manaphy could also be an event-exclusive, due to her connection with the sea and (according to the anime) Kyogre. Mew might also return, if they decide to include Faraway Island as an event-accessible area.