I think Evoss hit the nail on the head - you won't be able to compete with the big manufacturers, though will still need access to decent tools - a machine shop really. Then the case will have to fit a niche - like the LittleDevil cases (one of the only cases designed from the outset for phase change cooling) - and the niche has to have been avoided by the larger manufacturers.
Basically, this leaves two major options at each end of the scale (literally). Very small powerhouse cases, and at the other end, water/extreme cooling ready cases. Another start up manufacturer that's hit into the small end is Lan Gear, whose cases offer a very focussed set of features (large coolers, GPUs, small case, not much room for WC, or HDDs) that will appeal to a group small enough that the major manufacturers aren't interested/worried about that market.