Nothing ages faster than a great game that never got a follow-up. One strong release is all it takes for players to spend years replaying the same campaign, revisiting the same world, and wondering why nobody ever came back for round two.
Sometimes the story was left hanging, but that is not always the reason. In a lot of cases, the bigger loss is seeing a brilliant combat system, a wild setting, or a genuinely fresh idea get one shot and then vanish while safer franchises keep getting sequels.
Note: We've included a couple of games that actually did get a sequel, but we feel they need another.