then there are these Steam trading cards, which you can ... look at?
ET3D: They are not different that physical trading cards, which were around long before collectible card games, and far as I know still are more popular than them.
True, but are those actually of interest for adults?
There may actually be some sort of culture gap here. ;) I mean, I saw someone mention "baseball cards" further above. And I know we have annual releases for stickers of all football players here in Germany, which you can collect in an album. I did that myself when I was 10. But that's considered something you kinda grow out of once you realize that you're spending a lot of money for stuff that has no use and just gets replaced every year. I've never seen anything like this here in Germany that was addressed at adults. And if an adult was collecting those football stickers, it would probably look very strange. Not even in a "You play video games? Aren't those for kids?" kind of way, but rather in a "You are 30 and still spend hours playing hopscotch?" kind of way.