Basically look at what devs are scared of doing the most and you have your answer, shooter tutorials as well as third person games are a dime a dozen as an example so you should explore other niches
. RTS games
. CRPG party based games
. AI/Navmesh behaviour
. Driving games
. Space Sims
. Fighting games
. Simulation games
. Building games
. Shaders ( Especially fancy effects )
. Flight simulators/Flying mechanics
. Turn based strategy
This list is a hint for other would be tutorial makers by the way 😛 I'm going to be doing what I can to try and help out with AI I think and strategy games though I just post my stuff up and do a write up as video tutorials aren't really my style.