To get started with an idea like this, you really need a specific use case or customer who wants something. Then you build it for them. You've dumped a big list of features, but at the early stage it's important to only work on the ones which actually create value for a specific user or use case. Do your customer development first. This is a really ambitious project so paring it down is probably your best bet.

I do think "works on mobile" is a really great thing, few forums work well on mobile and it's definitely the future :)