Well, I actually JUST did an experiment to see what could be done from a complete zero on YouTube on my secondary channel!
As a long time vlogger (before I reformatted Internet After Dark) I had a great deal of reviews mixed into my classic episodes - and even to this day, they have some of the best retention and monetization return of all of my videos. But the two that are always at the top of the list in my Analytics were the two that I created based on Gravity Falls.
So I crafted a show around that. I used my interest in the show to do something more than just make a talking head video - I made a movement. I made a reason for people to interact and come back each week. I wanted to get people excited without a huge investment of either money or time - so I got clever.
I made a "secret society" for the show. A big part of Gravity Falls popularity is it's frequent use of codes and mysterious unexplained plotpoints - so I engage the viewer to submit their own thoughts on each episode's talking point. I created a community where everyone has a voice and nothing is shot down as incorrect.
But most importantly I created something of value that the audience would have to earn. I used a Google Form and encoded a question that the audience would have to crack and answer in order to get a download link to a membership card. If they contact me back, I add them to the list of members and raise their rank in the society if they share my video along with a personalized picture of the core download (In this case, a signed membership card). That in turn got me more views and more members and a wider audience.
The best part? People LOVE it. They love feeling like they're a part of something. They like that their hard work gets them something of value. I discovered and sought out a niche audience, and I landed it. I've been cracking at my main channel for years, and I've only just started to see growth - but this. I've yet to discover growth like this show has seen. I've netted 300 Tumblr subscribers, over 70 YT subscribers - and the roughly the same ammount of card carrying members, and my first video already has over 1,000 views.
I'm not breaking any records, but this is crazy growth for something under a week and a half old that started from nothing.
If you take anything away from this - just know that if you can create something of value that you can distribute digitally at no cost to you or your audience, people will love you for it.
Also, focusing on a niche is a great way to get a core audience quickly rather than trying to be all things for all people.