Once you finish Diablo 4's campaign, your main goal is to work your way up the World Tier ranks to earn better loot from harder monsters. Right now, World Tier 4, aka Torment, is the highest level of difficulty, but there's evidence of another tier in the action RPG's files.
Former Diablo Immortal dataminer DatamineARPG has found text and an icon for World Tier 5 that would fit in Diablo 4's existing difficulty menu. The dataminer said they found a marker placeholder that would point to the capstone dungeon you would need to finish before you unlock it.
DatamineARPG also found names for gems that aren't available in the game yet. Currently, you can craft five tiers of gems and then slot them into your gear for small stat bonuses. While the files have no information on a sixth tier, there are names for the seventh and eighth. Why would you need even more powerful gems? Maybe to survive World Tier 5.
Here's the current breakdown of World Tier difficulty:
- World Tier 1: No bonuses
- World Tier 2: 20% bonus XP, 15% bonus gold from monsters, increased enemy difficulty
- World Tier 3: 100% bonus XP, player elemental resistances reduced by 20%, increased enemy difficulty and variety, Sacred and Unique items can drop, Nightmare Sigils can drop
- World Tier 4: 200% bonus XP, player elemental resistances reduced by 40%, increased enemy difficulty, Ancestral items and new Uniques can drop
If World Tier 5 continues the pattern, it will probably be recommended for max level players (level 100) and could introduce an entirely new type of rare item. It's possible that World Tier 5 will be how you efficiently surpass level 100, too.
Given that Diablo 4 is fully a live service game, I think it's very likely that the endgame will expand as time goes on. The game's first season and associated battle pass will start in July, which will be the first step in understanding what direction Blizzard plans to take it in. Seasons seem to be where most of the game's new features will live, including new quests, and changes to how you level up each new seasonal character.
Outside of that, Blizzard hasn't said anything about further updates to Diablo 4 or whether level 100 will always be true level cap. It's only been a week since the full release and players are still grinding their way to max level. If we see an endgame extension, I'd expect that to happen much later in the year.