Absolutely nothing. :3 There's just a nifty certificate that you get, and you can use Pal Park in the newer games.
Chat with our AI personalities
For the shinno pokedex, it is given to you from Prof. Rowan in Sandgem town at his lab. For the national pokedex, it is given to you from Prof. Rowan and Prof. Oak in Prof. Rowan's lab after completeing the Sinnoh pokedex.
once you have completed the pokedex, i am pretty sure you talk to one of the prof.[oak or alm] and they give you a gift.[mabye national dex]
If you complete the national pokedex, go to visit Prof. Oak and he will give you a ticket to a place called foggy valley. You have to get a ticket because it is also an island.
You needs to complete the sinnoh pokedex and go to prof rowan's lab get national dex and the kanto region's prof comes in to give you national dex
You go to Prof. Rowan's lab in Sandgem town and talk to him. Their should be a cut scene where Prof. Oak comes and you should get the national pokedex.