#658 is more than likely to be Froakie's final evo, which leaves 8 more to #666. We know for sure that Fletchling is a route 1 Pokemon, since screenshots have shown it as low as level 2. We can assume that the Scatterbug line is found early on, probably within the first two routes, as it is most likely the Caterpie of this generation, and that the first gym uses Bug types.
Assuming that Fletchling evolves directly into Talonflame, and does not evolve further, and that it and the Scatterbug line come in succession right after the Froakie line, the Pokedex would look something like this:
That leaves 3 more Pokemon until #666. This is entirely my speculation, as there is really nothing to base it on, but the only other Pokemon that might be found within the first 2-3 routes that we know of so far would be the Pancham line, and the Litleo line. Let's assume that Pangoro does not evolve any further, and that both lines of Pokemon come right after Vivillon.
We can safely assume that the rest of the National Pokédex will start with the nine starters and a legendary probably preceding them, followed by butterflies, birds, rodents, dogs, cats, various insects and arachnids in any order. Below are two of the few possible outcomes: