Well as with any Pokémon game, certain Pokémon are missing from it. However since theres like 149 Kanto Pokemon and about 50 Johto that aren't obtainable, I will only list the Pokémon from Hoenn that aren't obtainable in Pokémon Emerald and therefore you must trade:
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Missing Pokémon in Emerald
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Surskit |  
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Masquerain | 
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Meditite |  
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Medicham |
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Roselia |  
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Zangoose | 
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Lunatone | 
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And those are all the Hoenn Pokémon Unavailable in Emerald