And now... a list of Pokémon from generations 1-4 that we all know will appear in generation 5 and some that will probably make an appearance.
Pichu, Pikachu, Raichu: It's so obvious that these three will appear and a carbon copy of the middle, it's not even funny any more.
Zubat, Golbat, Crobat: Annoying bats in caves are traditional.
Psyduck, Golduck: You just gotta have ducks.
Abra, Kadabra, Alakazam: The early Telporter is also standard.
Machop, Machoke, Machamp: A good bruiser is norm as well.
Tentacool, Tentacruel: Jellyfishing, jellyfishing, jellyfishing!
Geodude, Graveler, Golem: Rocks that you often find in caves as well as the bats.
Magnemite, Magneton, Magnezone: It's hard to say whether or not these three will actually appear in Black and White because while they did show up in Platinum in-game, they didn't appear in Diamond and Pearl.
Gastly, Haunter, Gengar: I think they have a good chance of showing up, even though they didn't make an appearance in Hoenn.
Goldeen, Seaking: Standard fish you usually catch while fishing.
Magikarp, Gyarados: No Pokémon game is complete unless it has at least one trainer in it who has only Magikarp on his team!
Eevee, Vaporeon, Jolteon, Flareon, Espeon, Umbreon, Leafeon, Glaceon: Again, like the Ghost trio, Eevee has a good chance of showing up, despite not being in Hoenn. Problem is, if Eevee does show up you can most likely expect two more Eeveelutions.
There are probably a whole lot more I could list, but the post would be way too long considering there is almost 500 Pokémon, at the moment, and every single one will be available post-game, but these are the ones I believe will most likely appear before taking on the Elite 4 and Champion.