Clean, simple, subtle, maybe kind of abstract. Not frilly. Less vibrant, more washed-out colors. I was thinking gray/white, but other colors are fine. No more than two colors. Some sort of leaf is OK, but not required.
This shirt shouldn't be a walking advertisement. It should be subtle, something that fans will enjoy wearing without feeling like they have to be representing the band while doing so.
See copperleafmusic.com for my current design style and to listen to my music. I make controlled, orchestrated music with emotional lyrics, mostly about love, loss, and memory. My stage presence is clean, quiet, and unobtrusive.
I use Alegreya Sans (https://fonts.google.com/specimen/Alegreya) on my website. You don't have to use that font, but if you're going for something similar to that font, it might be best to just use it.