I'm curious about their choices for filler Pokémon. While there are already roughly 580 different species (not counting any gen VI Pokémon, present legendaries or starters and their evolutions), we know that certain Pokémon are there because they're being retyped as Fairy Pokémon. I understand that, even in DPPt, Psyduck was just kind of there to be another Water Pokémon for the sake of mild diversity among the wildlife, but why Ledian and Swellow(assuming that it's true)? It just makes me think that it'll be really hard to track down certain Pokémon if they have too many dex entries.
Some are obvious, but some do indeed cross the lines of new Pokémon. For example, Machop-line has been the go-to early Fighting-type until Timburr and no generation until Gen V introduced a true counterpart for it, only Kanto and Hoenn had other early Fighting-types in the form of version exclusive Mankey-line and Makuhita-line and to a lesser extent Meditite-line, though the latter took this role more obviously in Sinnoh. Similarly, some Pokémon were introduced because their counterparts were non-existent, such as Nosepass which fills in the role of Onix as a wall for the first gym(it even got a part Steel-type evolution in the gen after). But there are stuff that cross the line with each other, such as Abra, which along with its (arguably) counterparts Machop and Gastly-line took the role of an early three-stager of its type, and in Abra's case one that is hard to properly train and obtain... even if we did have Ralts, which was meant to do Abra's job. Granted, you can obtain Ralts a bit earlier but still.
Some stuff on the other hand, are just ever present. Even though
Bulbapedia made a list out of them, it still doesn't include stuff you can catch in most games, such as Slugma-line, Pikachu-line, Shroomish-line, Girafarig, Ditto and others. Some of them actually have clear defined roles, for example Slugma is meant to be an Egg hatcher(though Litwick-line might one-up them in that area) and Ditto's obvious, but others, not so much.
Though by and large, B2W2's selection seems to be primarily based off fanservice than game balancing, which is rather obvious considering the promotions they have done. But with the hammering down of "You can catch old Pokémon!" ever since the first direct(as well as that whole "you can obtain Kanto starters" thing going on in Pokébeach leaks), this might be the case for X and Y as well. If anything,
the amount of old Pokémon we have been showcased has reached 57, which is quite impressive.