almost all of the new pokemon worth using are either locked to kanto or tied to incredibly obtuse mechanics like the random tree pools or the bug catching contest, you literally can't get a pokemon that can learn moves effective against flying types before the flying gym, etc.
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Also, you can get Geodude very easily before the flying gym, you can trade for an Onyx in the same city, and the gym is like level... 10? And flying type. All you need is tackle.