You don't need much to get an MVP together. Check out SimpleWebRTC for a free, open source video conferencing solution.

I just built a live video platform using a variation of that software (the next generation, Otalk, which is more complex) and found it really simple to get things breathing.

Once you have a prototype, just get people using it. See if someone will pay for it. Keep asking. Run targeted ad campaigns where your target market exists.

You don't have to start with a well-defined niche; see if a group buys in, and your niche may identify itself.

I have a lot of experience with both realtime video and validating/building/marketing new products and services. I'd be happy to offer some insight if you're interested.

Good luck!