Two main things:
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UI! I mean the game UI not the engine UI. No matter how good a game engine is, it always seems that they sort of phoned in the HUD system. Keep in mind I come from a React/Angular background. Currently I wrap all my games in Angular, because it’s the best for building a UI
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Module/Component/Things library. Unreal is a good example of how this is done right. You want a 3rd person controller, there’s a prebuilt blueprint for that. You need a vehicle controller, there’s a prebuilt blueprint for that. I know this is something that Unreal has gathered over time, but it’s a very nice feature of the engine.
Just my two cents :)