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Something like the Uber Bosses in previous titles paired with a visual nod to Diablo 2, or introducing an item like the Horadric Cube would go a long way to satiating the fan base until the loop of seasonal ladders with leaderboads is up and functional. Whether it is something entirely new for Diablo 4, or just expanding on the least fleshed-out features, Blizzard can not afford to get by on the bare minimum.
The less it does to solidify its long-term players now, the more it will have to do with a larger-scale expansion down the line to claw them back. In terms of player retention, Blizzard will want to spend a penny now to save a pound later. Regardless of what season one entails though, Diablo 4’s initial offering at launch is an impressive one, and bodes well for what is to come.