The Romans, if you consider 'diversity' to mean more than just skin color. Their empire brought together:
Latins
Italians (NOT the same as the OG Latin Romans, peoples like the Samnites, Sardinians or Etruscans spoke different languages and organized themselves quite differently compared to the Republic or Principate)
Greeks
Gauls & other Celts (Britons, Rhaetians, Celtiberians, etc.)
Iberians (not to be confused with the Celtiberians, who were a distinct populace from pre-Indo-European Iberian peoples like the Turdetanians and Aquitani/Basques)
Illyrians
Thracians
Dacians
Africans (the North African peoples weren't black - those called Numidians/Moors/Libyans were Berbers, the Carthaginians were of mixed Semitic and Berber ancestry, and the Egyptians were their own thing entirely)
Remaining Anatolian peoples, ex. the Isaurians
Syrians (NOT Arabs, they were Aramaic-speakers and their descendants are today's Syriac-speaking populations - Arabs didn't move en masse into Mesopotamia or the Western Levant until the 7th century Islamic invasions)
And even some Jews (Titus' second-in-command at the siege of Jerusalem was a Jew for example) and Germans (Rome's German provinces roughly covered modern Belgium & the Rhineland, and one of the men who could've saved Rome in its twilight if he hadn't gotten assassinated by the Senate was Stilicho, a Romanized Vandal). The Romans' downfall had more to do with constant backstabbing & civil wars and less with 'diversity', even the migrating Teutons generally took to Romanitas like fish to water.
Aside from the example of Stilicho up there, the barbarian successor kingdoms like Francia & Gothic Spain quickly forgot their native tongues in favor of local Romance dialects and adopted Roman law, customs & Christianity pretty quickly. Even Alaric, the guy who sacked Rome in 410, was trying to get high office in the Roman army and only sacked the city because the Senate massacred his soldiers' families after their coup against Stilicho and then kept trying to backstab him during negotiations despite (or because of) him constantly destroying their armies in battle.
The Romans, if you consider 'diversity' to mean more than just skin color. Their empire brought together:
And even some Jews (Titus' second-in-command at the siege of Jerusalem was a Jew for example) and Germans (Rome's German provinces roughly covered modern Belgium & the Rhineland, and one of the men who could've saved Rome in its twilight if he hadn't gotten assassinated by the Senate was Stilicho, a Romanized Vandal). The Romans' downfall had more to do with constant backstabbing & civil wars and less with 'diversity', even the migrating Teutons generally took to Romanitas like fish to water.
Aside from the example of Stilicho up there, the barbarian successor kingdoms like Francia & Gothic Spain quickly forgot their native tongues in favor of local Romance dialects and adopted Roman law, customs & Christianity pretty quickly. Even Alaric, the guy who sacked Rome in 410, was trying to get high office in the Roman army and only sacked the city because the Senate massacred his soldiers' families after their coup against Stilicho and then kept trying to backstab him during negotiations despite (or because of) him constantly destroying their armies in battle.