I'm not familiar with ElasticSearch but everything else you mentioned is easily accomplished with Elgg, most of it out of the box.
Why reinventing the wheel? Most of the things you mentioned are available in Elgg our of the box. All you need is to extend some functionalities of it. Developers like izap has implemented Elastic search with Elgg. Google will provide some information on this.
There are already plugins available which exposes youtube other video sharing site's APIs.
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