The announcement is, more or less, a love letter to the franchise. It's being called Pokemon Rainbow, a 3DS game that encompasses the settings and Pokemon from generations 1-4: Red and Blue, Gold and Silver, Ruby and Sapphire, and Diamond and Pearl. You can take my word for it that the graphics are done up very nicely. The perspective is similar to what you'd expect from Pokemon games, but the camera is much more dynamic. The graphical style is characterized by very clean, dark lines. Almost cartoonish, but not quite. Battles look radically different, and move away from the use of pixel art. Though it seems that the interface is still made of pixels. The battle that it shown is between a totally new trainer and a wild Weedle. The trainer uses a Pikachu.
I'm not privy to any story details, as I only know what will be announced tomorrow. The story is concurrent with Pokemon Black 2 and Pokemon White 2 in terms of timeline. There are over 20 new Pokemon, and two will be revealed tomorrow. One is an evolution of an older Pokemon Eevee, called Draconeon. The other is a totally new Pokemon, called Verbill. It looks like a rodent, much like the Pokemon Patrat, but has a sort of pirate theme going on.
A Wii U app will be launched along with the game, which will be announced tomorrow. However, all that will be said is that the app will be addressed "at a later date". The logo for the app features a globe with Pikachu, Munna, Treeko, Cyndaquil, and Piplup popping out from various countries.