Icons over targetted player
 

Sometimes in a big group of players it is tough to see which one you have targetted. The only thing blizzard has implemented is to slightly highlight (increase brightness of the target).

Is there a way to automatically place a custom icon/marker over the targetted mob or player?

If you use nameplates it will change color when you select a target. If this isn't apparent enough for you, you could look into getting a nameplate addon (i.e. Tidy Plates) and add additional visual features to see your target.

I'm pretty sure Tidy Plates' Threat Plates has the option to add ">>" and "<<" to the edges of the nameplate to make it more visible.