If you have ever heard of OfficialDuckStudios, he is by far my favorite game commentator. It would require you to be VERY vulgar and random. But he managed to get a partnership with machinama from it. But besides vulgar comedic commentaries, you could also do the classic actually helpful ones.
If you mean for a game itself that you could commentary, I would recommend something weird like "The Crimson Room" or "Happy Wheels."
GOOD LUCK!