Custom icon for Collection?

I realize I'm chiming in a few months late, but I want to second macmaarten's request. The current solution described by @ag_maryam -- that "vault collections get a grouping of icons from the included vaults" -- seems inadequate, because of these grouped icons, only the frontmost is at all visible. All that's visible of the icons in the background is the faintest hint of a circular outline. It's certainly not enough to let the user know which icons are back there, and it's barely enough even just to indicate that there are any other icons at all.