Might have been mentioned already, but since everyone is talking about megas right now, I'll bring it up anyway.
Assuming that the information on
this page is true, Salamence's pokedex number in ORAS is now 198, but Salamence was 189 in the original Hoenn dex. This means that there are at least 9 pokemon added to the Hoenn dex.
It is also worth noting that there are 9 pokemon from the fourth generation who are either evolutions or pre-evolutions of pokemon who were in the original Hoenn dex. These pokemon include: Gallade, Probopass, Magnezone, Budew, Roserade, Dusknoir, Chingaling, Rhyperior, and Froslass. This seems like a perfect fit, 9 new pokemon/ 9 new spaces, until we take into account that since Mega Lopunny is now a thing, we can deduce that both Buneary and Lopunny will be available, bring the current total number of new pokemon up to 11.
Therefore, there are several possibilities regarding additions to the new Hoenn dex:
1. Only the fourth gen pokemon who are related to pokemon from the original Hoen dex will be added to the new dex. Meaning Buneary and maybe some others will only be available after the national dex is obtained. (Possible, but all of the "related" pokemon are obtained through the use of evolving with items that didn't exist yet, bred with incense that didn't exist yet, or evolve in places with high magnetism which I don't recall Hoenn ever having a place like that. It is all dependent of whether GF gives us these items/locations pre or post E4.)
2. The Buneary line and some other pokemon (possibly some but not all from the "related" list) are added to the new dex and therefore available pre E4.
3. A combination of 1 and 2 with some pokemon after Salamence in the dex. (Though since the only other pokemon after Salamence in the original dex is the Beldum line and the legendaries, this seems unlikely.)
4. The information I linked above is false and you should disregard everything I just said.