Mainstream stuff works; challenges, playthroughs, battles, walkthroughs (detailled and informed playthroughs), reviews, speculation videos, top tens, etc. They all are found on a lot of channels, because people like watching that stuff. More importantly: that's what the masses like to watch.
It's kind of a blessing and a curse at the same time. You might attract more viewers if you include this content into your channel, at the same time there's already tons of other people doing the same thing, which means that your channel might not become that much more popular afterall. Optimum would be if you found a niche that's still unexplored, something the others haven't done, yet. However, that's going to be difficult and not everyone is able to spot those niches, hence why they go for the mainstream.
It all depends on where you want to go with your channel. If you want to grow substantially larger, then you might need to find niches, but if you just want more content, then choose one of the things I mentioned in my first paragraph.