I was thinking, since Chickenfist made a great mod recently, that so maps could be found easily in the Map Vault, to change the ? for HL2D maps to some sort of 2D with a background color instead, for example, how Half-Life maps use the HL symbol with orange background, so maybe 2D with a white background.
If you haven't seen HL2D yet, it's in the 'Maps and Mods' section of the forums in 'Flat-Life HL2D'.
That's just my idea.