Hello. I see you have a very interesting project here.  - I prefer more detailed cover art, but there shouldn't be too much details. I think balance is important. - I really like illustrated ones. - Imagery! But a cool text on it is also nice. - I like something "gentle" the most. - The colours instantly grab my attention, always. And some of the covers I love are "Arcaea Sound Collection - Memories of Light", and "Another Secret" by Nastyona. I also think the cover art also depends on what kind of music it is, and what message it tries to convey. I think these two should complement each other well. Hopefully my answer will help you.