One of the most important things to keep in mind when developing content for your channel is not just the size/potential of the demographic, but also the uniqueness/usefulness of your skill/personality/whatever the hell you are trying to provide is to that community.
With GizmoSlip, we focused on standardizing drop tests for phones and devices because we realized that there were huge markets of people out there who are worried about breaking their phones. We then tailored our videos to be as short and concise as possible so people didn't have to waste time. Then, and this is more important than you might think, we actively pursued other tech venues who had already established audiences. We developed PR releases to put out to all the magazines, tech blogs/websites, and tech social media sites as we could. We often wrote individual writers and bloggers about our videos and did our utmost to make sure that we were doing our stuff in a way that no-one else had done. As a result, we got posted by Engadget, Kotaku, Gizmodo, Tech Crunch, etc on a regular basis.
In terms of useful skills, we invested in some high-speed camera rentals to gain experience in slow motion so that we could easily leverage that as a skill that almost noone had in the YouTube realm. Because of that skill set, we were able to leverage ourselves into collaborations with YouTubers that were huge even when we were small. When we only had 10k subs, we were able to collab with FinalCutKing, JacksFilms,Nigel Alexander, etc. Millions of viewers saw our work through these collabs and we gained tens of thousands of subscribers through them.
The point I am trying to make is that no matter what field you try to dig into, it is WAY better to be an expert in something niche than to be a jack of all trades and create generalized content. Build your market around something that you can grow quickly so that down the road you can leverage yourself into other markets as well. For me, tech durability and slow motion were my fields of expertise, but now that I have an audience I am able to break out into more generalized film ventures and have an actual leg to stand on.
