I'd just go for a candlestick or two on one side and maybe one of your photos on the other -- let the deer have the rest of the stone space.

Take off everything including forehead and let the stone bask in its glory.

I ment deerhead...

OK- let's just say this is quite the busy location. What is the gated area for?

Get a larger mantel or take it off .

I like your idea of leaning art. I think instead of the mirror, a tad larger art piece (or photograph) with the other leaning with it. Then, a simple mantel clock on the right side of the mantel. Three things only...2 art pieces, (one large, one medium), the clock on right, then the deer head above. A vintage looking mantel clock will look right with the deer head. Sort of like this arrangement only both art leaning on the middle left and a mantel clock on the right side.

so, we had this mantle custom made and we are upset that it is too small also but we can't remove it. there are lights under as well as above it. the gate is for babies!

If the deer is staying, I wouldn't do anything else (well I'd remove the mantel but you say that's not an option). If the deer can go, I'd put some nice tall objects on the mantel. I don't get the lights under the mantel -if you can't remove them, just leave them turned off.

it's an elk....and the under mantle lights look pretty nice at night. but yes, unfortunately there in no stone under the mantle. it's built in.

lights

OK, we'll work with what you have then, but the lighting is all wrong. Turn the under-mantel lights off, and adjust the upper light to shine on the mount and not on the wall. Then keep your decor under the shoulder of the fireplace and about the width of the firebox itself.

Would it not be possible to somehow augment the mantel? Add some wood to either side?