Or it’s something wont happen in near future?
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There are no plans currently of going open source. We are working on low level extensions though and Editor 2 is built with extensibility in mind.
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Are there plans for a plugin marketplace or otherwise?
I hear extensions and I immediately think marketplace 
No plans for a marketplace, but certainly for people to easier share/discover things. We have content sharing now called libraries, but they do not let you extend the engine with high-perf code, native extensions, or editor/pipeline extensions. That’s what’s missing.
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