I'll preface this with a disclaimer that I'm not a professional designer by any stretch. I have a couple of ideas, however.
For starters, I'd recommend you landscape the bottom of the lake to a general style that you enjoy. Make it either larger or more symmetrical, in line with the design above it. Making it bigger also benefits the quality of the catches! Next, finish what you had in mind for the building above the lake. Stick to Pearlstone Bricks as your theme. Some hallowed furniture would look nice.