I've seen a few tanks in-wall like yours is Christy and what really impresses me is when the rock slopes up on either end with low rockwork or sand showing in the middle...not quite symmetrical though. Only an idea I'm sure whatever you have planned will be bomb =)
Here's a fairly densely packed scape that illustrates what I'm talking about although this particular tank is not in-wall. The design could always be thinned some.
http://reefkeeping.com/issues/2006-04/totm/index.php