I hope that they have a nice distribution of Pokemon, so that if you have a Pokemon of a certain type you can't just cruise through the area. In the beginning of BW2, literally EVERYTHING was weak to fighting (aka Riolu), and also almost all of Team Plasma's Pokemon were weak to fighting or ground. I used a Riolu once and a Drilbur in another playthrough and the game became soooo easy.
I don't think they really need to even make more Pokemon of a certain type, they just need to include more from past gens in the game, there are plenty.
For instance, it's not so much that the national dex needs more waters. I don't battle competitively, but I'll take the word of those who do that there are already too many waters. However, UNOVA had too few waters, which was why that was mentioned in the OP. However it is that GF decides to deal with it - by introducing entirely new ones or simply using more from the past dexes - there needs to be more waters in the new region than there was in Unova, just as there needed to be (and were) more fire types in Unova than there were in Sinnoh.
I have no idea what types might be necessary to fill out and better balance the National Dex and can't address that. It and the regional dexes are different subjects, and it seems that much of the disagreement on this thread has come from treating them as if they're one and the same.