I think the problem is that if you have to sit down and think for a long time about what series to make, that you're only in it for the money; and I think that is a problem with a lot of beginning YouTubers.
The secret behind having a successful channel is being interesting yourself. If you can sell both yourself (figuratively of-course) and the game simultaneously then it will be a successful channel.
ill help man, anything to help a fellow minecrafter in need. Im an amazing builder so long as you have bukkit and world_edit installed on your server. Or not bukkit but creative! anyway please invite me i would love to help!