Spectators get a map ping when heroes attack each other on the mini-map. This would play very similar to how the game notifies a team of a tower assault, but specific to spectators. This will draw the attention of viewers when watching games instantly to the action. Or during replay to skirmishes they may not have noticed originally.

Pings would be colored according to the aggressive Faction or Player.

Pink attacking Blue creates a Pink ping on the minimap for a short time with a quick sound(that can be toggled off). Even without going to the spot, the spectator knows what is happening and can decide whether they want to inspect it or not.

This could also be configured to display as faction colors for Hellbourne(red) and Legion(green).