You get your pokedex upgraded when you defeat the Elite 4 and Wallace. Professor Birch will call you to the lab, and he'll upgrade your normal Hoenn pokedex into a National pokedex! Now you can explore more parts and capture new and different pokemon. I believe the pokemon that you are able to catch after the Pokemon League with your National pokedex would
be Sudowoodo, a rare tree pokemon you encounter in the Battle Frontier. Use the Wailmer Pail on him to make him attack you. You can catch him quite easily since he's only about lvl. 40. You can also go to the Safari Zone and capture new Johto pokemon. This is possible because it gets upgraded after you beat the Elite 4. Chikorita, Cyndaquil, and Totodile are possible
Johto starter pokemon if you complete your Hoenn pokedex. There are about 200 Hoenn pokemon for you to look for. You can also catch another rare pokemon in the Battle Frontier. It is called Smeargle, and you catch it in the Artisan Cave, where you can surf to after capturing or defeating Sudowoodo. Smeargle is an exceptional pokemon to have.