They have said they won't do that because it places an extra burden of knowledge on players as they not only have to know what all the champ splash icons look like, but all of their skins as well. Add in the fact that shrinking the icon that small can make it hard to make out.
An example I remember someone saying: "you see a fiery symbol headed towards you on the minimap, and you assume it's the Brand, so you think you have more time or can deal with him. Turns out it was Volcanic Wukong and he was there a lot faster and was able to kill you easily"