Today’s video was a bit rambling, but the Too Long/Didn’t Watch version is that your cover has to serve a single primary purpose, and that is to get the viewer’s attention. Once you have their attention, they may click through to your product page, and there you have much more space to engage their interest and answer their questions before they buy your thing.
There are two ways to grab their attention. The first is with something really evocative of your thing’s content. A horror-style cover on a horror supplement or game, and funny cover of a comedic game, and so on. The other option is to focus more on your own branding than that on the individual supplement. This is what the fixed covers of Hero Kids do; this is what the consistent cover structures of FATE, Cyberpunk, and even my own slanty 3d mockups do.
Obviously, I think the branded way is the better of the two options, which is why I chose to go down that route. The advantage is that every thing that I release reinforces my branding. Now when anyone in my niche sees a slanted 3d book mockup they instantly know that it is one of mine, and if they have bought from me before then they pretty much know what they are going to get.
Seafoot maps are of a certain style and quality, and once you know that, as soon as you see their ribbon and wax seal, you know who made it and what it is likely to be like.
Hero Kids covers are the extreme form of this. They are literally identical. They do not even carry the title of the book.
Once you are past the branding stage, you probably want the cover content, and the color scheme to fit your content. Be wary of stock art that simply does not make a good cover. I fell for that pitfall with my Savage Worlds title. I could have chopped the chap’s head off, but it would not have been a good look. My only option was to fit my wording around the image.
If I had to pick one thing as ‘best practice’ I would say include at least one person in the foreground with a clearly visible face, and an expression that fits your content. A scared person for horror, laughing for comedic and so on. But make this person clearly discernable, not some tiny figure that could be anyone. The goal is to engage the viewers emotions and for that to happen they have to see what is going on.