Sorry for my ignorance but are there many games out there which you can create and sell game assets for? And have you had much success selling assets for specific games? And if so what were those games?
Steam games with a workshop are proooobably your best bet. I haven't looked deeply enough into those to know many that allow paid creations and how they handle it. I personally only know much about DotA and Warframe, and neither of those are great to mod for because 1, they both take an obscene cut of the earnings…