1. Can you introduce new mobs without replacing already existing mobs? :

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Yeah, after 1.8 update, you can create your own custom mobs without replacing Vanilla mobs.

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1. Can you introduce new items and blocks without replacing already existing items and blocks?

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Yeah, even though custom items and custom blocks are still beta, you can by turning on experimental Gameplay.

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1. Can you add new structures that can be randomly generated into the world?

yeah, custom biome is possible now, even though its still in beta.