I quite like Wintersong's recent set of avatar pics with element runes on the frames. The most I've seen on one pic, though, is four elements. Which got me to thinking...
If Elemental has an analog to the MoM spell Call the Void, it should require all 5 types of mana. (Call the Void wrecks an entire city, seriously maims or kills any units in the city, and corrupts surrounding land with the 'fallout' from the destruction.)
Any thoughts on other spells that would need the Full Five?