my list would be
1 - really really really big because its always cramped under there. Oh, or extra cabinets nearby. mmmmm maybe just an entire fish room attached to the stand.
2 - as mentioned already - ability to get things out from under it and dry under it.
3 - as mentioned already - attachment points that are waterproof on top, sides, and back. Need to hang a fuge light? check. Need to add a reactor? check
4 - I love the cat stand! I was thinking a bench that flipped down. I have a step stool next to the tank so I can reach the bottom.
5 - tool / towel / supplement storage. I've seen some with the metal kitchen baskets stuck to the door that looks good, but I would be afraid one would fall into the sump. So maybe just attachment points on the door too.
6 - worklight that is switched to on as you open the door.
7 - really cool exterior doors. I'll let you decide if that is metal, barnwood, whatever.