Is that whole where the return pump goes? If so I would go with a much bigger return area. Evaporation, is going to empty that pretty fast. I see you have an ATO, but if something goes wrong with it, it doesn't leave much room for error.
Also, a three baffle bubble trap instead of the section with what I think is a sponge shelf in between the skimmer and return would be more effective. <---not the best sentence ever written!
You could also try to incorporate a bubble tower for the drain from the tank.
My gut feeling is you have some great ideas and nice implantation, but just need a larger space in which to work. If you took your basic design increase the size by 2 or 3 times and put in some bubble traps I think you would be happy.
I have to say nice skimmer!
Can this sump take the water needed when you turn of your return?