I'm not keeping up with video games these days that well, but for whatever reason, I keep watching Rust videos lately and I've never even played the game (I played other survival games so I guess I'm just vicariously living through other people's experiences right now).
Anyway, niche selection is critical for new channels. I can't tell you what's better, but as a rule of thumb, it's always better to go for the sub-niche of the sub-niche of the sub-niche etc. - just deep enough that you don't have to compete with thousands, millions of other channels, but not too deep that there are only 10 people who would even be interested in watching your videos.
Take this advice with a grain of salt as this is mainly my opinion, but based on new gaming channels that I see take off lately, focus on one platform and one game at first. You have better odds that way instead of going after variety right away. Treat each video game as it's own sub/mini-niche. You certainly can dabble in other games/platforms from time to time, but keep up with 1 game.
Keep an eye on the trends too. How many players play the game - is the game getting more new players, stagnating or even slowly dying? I was a Planetside 2 (PS2) addict for a good while - I used to watch some PS2 YouTubers as a result. I checked some of them out recently, almost all of them are no longer uploading PS2 content. Some left, others who are still uploading have transitioned into other games and their channels have grown since the last time I watched their videos.
Me personally, I'm into tabletop RPGs. I make D&D 5e content exclusively and it works for me because more and more people are starting to play every day. However, if I notice the game slowing down, stagnating or even losing popularity, I'll most likely consider expanding into other tabletop RPGs, probably Pathfinder 2 as that's the one that gained a bit of ground on its own recently.
So, basically, treat this is a business/entrepreneurial endeavour, keep up with the trends and most importantly, upload new interesting videos as much and often as you can - you can't grow if you don't upload, so keep cranking.