Well, there is a whole new third generation of pokemon, plus some from the past two generations. There is also the safari where you can catch a whole bunch of 1st and 2nd gen pokemon.
In the Hoenn Dex, there are 200 pokemon. Kadabra, Graveler, Machoke, Clamperl and Seadra don't evolve without trading, so you can't get Alakazam, Golem, Machamp, Huntail, Gorebyss and Kingdra without...
Regirock, Regice and Registeel (look at the links to find how to catch them). Latios or Latias (depends on which color you choose when asked). Groudon, Kyogre and Rayquaza.