Today an official announcement was made for a 3DS Virtual Console release of Pokémon Crystal. This means that Pokémon from all main-series games can be legitimately transferred to the latest games. Back when Pokemon transitioned onto the Gameboy Advance system, Pokémon from Gen 1 and Gen 2 cartridges weren't legitimately obtainable (via transfer) due to limitations in the link cable design. The release on the 3DS Virtual Console will take place on January the 26th, 2018. In Europe there will be a physical release with the download code in the box, while in Japan there will be a physical release with a magnet based on Crystal's Cartridge, a poster with all the Pokémon released in gen 2 and with a gen 2 type compatibility chart, stickers, and also a download code. There is also a separate download code available in Japan.
The Japanese version holds the possibility to get the GS ball in Goldenrod City's Pokémon Communication Center, but only after beating the Elite Four. For Europe and America releases, while the English trailer announced that players will be able to encounter Celebi, there is no hard confirmation detailing whether it will be done via the GS ball at this point.
We first found out that Pokémon Crystal would likely get a Virtual Console release, as patches supporting Pokémon Crystal was found bundled together with Virtual Console's Pokémon Gold and Silver, as discovered by our well known member @SciresM (tweets as seen below). Later, when Poké Transporter got updated for Gold and Silver support, he discovered that Poké Transporter also preemptively supports Crystal, despite it being unreleased at that point.