Recently, I downloaded the Pokémon bank And after using it about twice, I got a Celebi out of it, which I was very pleased about. I was wondering, can any other Pokémon be obtained this way, or is it just Celebi?
The Pokebank Celebi is a time-limited event release for using the Pokebank before September 30, 2014.
Whether Nintendo/Gamefreak intend to release other pokemon by means of the bank rather than traditional means like Wifi wondercards or in-store events is unknown.
You will be able to get any Pokemon that The Pokemon Company gives to you via the Pokemon Bank. These are usually timed events and you will have to keep checking a source of information to find out when these events are.
So far, Celebi and the three Johto Roaming dogs (Raikou, Suicune, and Entai) are the only Pokemon to have been distributed this way, however they have all ended.
No other distributions are planned at the time of writing.
Don't forget you can also recieve Pokemiles or Battlepoints as well just from using the Bank and playing the games.