Postgame Mobs

Similar to games like Terraria or Dragon Quest, it would be interesting to introduce stronger enemy variants after defeating the Ender Dragon. These could be Rarer more powerful versions of pre-existing enemy types.
If that is too difficult to add, you could make it so regular enemies spawn with certain enhancements more frequently. Some sample ideas could be: After Defeating the Ender Dragon:

• Charged Creepers spawn naturally.
• Skeletons and Zombies spawn more frequently in Fully Enchanted Armour / or / Can now spawn with Diamond Armour.
• Spiders spawn with Potion effects more frequently.
• Zombie Pigmen spawn in greater numbers.
• Evokers/Vindicators may spawn alongside Pillagers outside of Raids.
• Endermen spawn more frequently, especially in Strongholds (they have been freed from The End).

These are just a few ideas to begin brainstorming, but feel free to add suggestions.