There's so much to do and so many places to go, you'd never get bored. I've played over a thousand hours and still find new things every now and again.
Impossible to answer. Even though there are 2 or 3 games that I can think of that I know will last for a year or two once I start them in the distant future, I cannot choose one game to play for the rest of my channel history. I move on from one game to another, an adventure needs a beginning and an end in my case.
Warframe. It's a really unique free-to-play game that keeps updating from time to time, and the devs are quite attentive of the game's players, so YouTube could be a way for me to communicate with the devs about the game.
If I could pick a Franchise, CoD no doubt simply because it's what I'm personally passionate about and it's the scene I want to be apart of.
Otherwise, GTA V for PC xD