Announced Events So Far:
"Use #Pokemon20 to share your favorite Pokémon moments on Instagram, Twitter, or YouTube! Our favorite #Pokemon20 submission may get something special, so keep an eye out!" -cited from pokemon20.com
The original game was released on February 27th 1996 in Japan, the official twitter account announced that the 27th of this year will be dubbed Pokémon Day! Maybe if we're lucky we'll get a surprise announcement of some kind?
If you've been trying to collect every single Pokémon legit (no cheat codes, GameShark, or modern day apps) since you were a kid, The Pokémon Company is here to help. Every month from February to December 2016, GameStop and the Nintendo Network will distributing a new mythical Pokémon starting with Mew! Check here for more info.
Tons of new merch, 20th Anniversary merch too! Here & Here
New Pokémon Trading Cards, "Generations" themed decks and individual cards. Word on the street is that Japan is getting reprints of the original set! Check it out here. I would love it if they reprinted the North American cards!
A re-release of the films Pokémon: The First Movie, Pokémon the Movie 2000, and Pokémon 3: The Movie on iTunes, Amazon Video, and Google Play. I don't see it on the official site, but I'm seeing a Blu-Ray 3 pack of the movies as pre-orders on various sites.
Lastly, what I'm most excited for ... the re-release of Pokémon Red, Blue, and Yellow Version via Nintendo 3DS' Virtual Console! Red and Blue will have the original graphics and music while Yellow Version will sport some enhanced color graphics. Check here, here, and here for more info. Nintendo of America is also releasing a special edition "new" 3DS bundle. This will have Red and Blue Version pre-installed, 2 cover plates (similar original Game Boy box art of Charizard and Blastoise on cover plates), and a download code for an exclusive 3DS theme. Japan and The UK (minus Green Version) are getting some fancy 2DS console bundles! Check it out here. I was really hoping Nintendo/The Pokémon Company would release Green Version State side, but I don't think it's going to happen. I can't complain, this year is going to be amazing!
FEBRUARY 2016 UPDATE: Pokémon Sun & Pokémon Moon #PokemonSunMoon https://www.youtube.com/watch?v=hmzDL1l3Fag