Like Cookey said, it depends on what you want to do. It's better to focus on what makes you happy rather than what's popular. Let's plays are always a good starting point that you can continue with. You can kind of add your opinions on the game you're playing into those kinds of videos too. Another good kind of video you can do is highlights, like if you play a long game or a multiplayer game you can record a long session(s) of that & take the funniest or scariest or whatever clips from that.
I tend to make a highlight video after playing a long game & a couple shorter games with the scariest & funniest moments from those.
A good time to try to stick to is about 15-20 minutes. Unless the game is a long one, then you could get away with making each part longer that way you're not uploading part 50 & losing the viewers' interest.
Multiplayer games seem to continually be pretty popular. Stuff like GTA5, Gmod, etc.