If you're not maxed out, you're really close. Those clowns will get quite a bit bigger than they are now and take up a lot more visual real estate. They also tend to get very territorial as they age and they'll easily claim a whole 15g as their own. I have a pair of occ. clowns in my 60g and they pretty much run the show there. I tried to add a couple of fish a year or so ago and had to remove them. The clowns were bullying them to the point that they were going to die soon. They don't bother the damsel and firefish that have been in there with them since they were small, but no new fish are allowed. A 15g cube is probably the same size as the rock overhang my clowns have claimed as their nest. The rest of the 60g is just home territory.
If you can add anything (which I'd strongly consider not doing in your shoes), it's a small goby. I don't think that would be irresponsible of you, but it's close. I think you'll find that the tank looks a lot less empty as you get coral and it starts to grow in. You won't feel like you need fish to add interest because you'll also have coral.
