The best replacement for diocese is archdiocese.

The best alternatives to diocese are rated as following:
★★★★★  archdiocese
★  prelacy
★  parish
★  national diocese
  diocese

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Speaking about diocese, one probably shouldn't forget about bishops.

Other search terms related to diocese and their ratings are:
★★★★★ bishops
★★★★★ bishop
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Google Trends

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diocese
archdiocese
prelacy