3. Set Items Tied to Group Progression
With set items rumored to Diablo 4 Items eventually return, Diablo 4 could tie some set pieces to cooperative achievements:

Defeating the Escalating Nightmare Dungeon trio as a party on a high tier.

Completing “No Death” runs with all four party members.

Participating in a rotating group-focused seasonal event.

A 2- or 4-piece bonus could grant team-based bonuses, such as:

“When all party members wear 2 pieces of the Sentinel’s Resolve set, damage taken is shared evenly across all wearers.”

This would promote team-based build design and cheap Diablo IV Items coordinated party gear decisions.