I've considered this in the past, too. About one third of what I watch on YouTube is people playing video games and having a good time doing it. If you were going through with it, you should absolutely add your own voice to your recordings. That's the best way to differentiate yourself from, say, megamanx_gamer57 playing the same game as you.
As far as full walkthroughs go, I tend to have more fun watching people play sandbox style games where there is no real linear path to follow. Speaking as a viewer, it feels more organic and unpredictable to watch something like that versus a Mega Man X runthrough that's pretty straightforward and I already know the lay of the land (walk right, kill enemies, collect power-ups, jump through boss door, kill boss). Even games like Skyrim or GTA, where I know the world pretty well, is more interesting to watch because I don't know specifically what the person or people are going to do. Unless there's something spectacular about a linear game runthrough, like a speed run or a funny glitch, it's a video I would skip over. Hopefully that made sense.
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