The rpg can be a great asset to keep defense but the fact of the matter is, its a pain in the arse . You can never tell how far to aim it to get a hit on a target . If there was a UI addon to Wintergrasp that put like a target on the ground that gave at the very least a target on the ground of where the friggin thing was going to hit and the area of damage it covered would be extremely helpful to me. It's so annoying id go as far as a donation to that programmers paypal acct for it.
Addons can't interact or draw anything on 3d world.
edit: While it is probably possible to somehow load WSG 3d map into an addon and then make some approximations based on character and camera position, then draw target spot on overlay, it is not trivial at all.
Similar addon example and it's limitations explained on description (some limitations has workaround with remapping zone maps within addon and that would be pretty much required for rpg stuff): http://www.wowinterface.com/download...16249-AVR.html