I'd say to succeed, you have to offer something no one else is doing. If that's the ability to make a custom form fitting case for any instrument, then that would be a service people might be interested for their rare/valuable custom instrument that doesn't fit in a standard case. (or the cases that are available that fit it, don't meet the requirements of the instrument owner) This would be a high end, 1 piece at a time business. High profit margins, but very time consuming, and people expect high quality work. You wouldn't get many customers, but the customer you would get hopefully would keep you busy and put nice money in your pocket.
(you could offer a large (endless?) variety of inside materials and even outside materials as well, etc.)
The way other case companies have tried is to offer a standard item that can be made in great quantities, and found the cheapest way to make it, so that they can be a low price leader in the market. There's a lot of people that want a case, but will spend the cheapest amount on it that is possible. Quality is less of a concern, as long as it functions. (this last part is important - a case that falls apart, breaks quickly, or damages the instrument in the end due to it's use is not a good product to sell, even at 1/2 the price of the nearest competitor.)
If your plan is to make a case that is on par with your typical mid-level Gater/Fender/SKB case, and sell it for about the same price, then that's a tough sell. Consumers in that price range are looking for reputation, and building one can be tough. Kinda a chicken & egg sorta thing. Your case is as good as a Gator, and $10 less, and made in the USA - but no one has seen one to compare the quality, so they choose something else. (also no one would likely know it's even available since the majority of people shop for their gear at major distributors -
Musicians Friend for example)
One option to get you started would be to get a paid membership here (gold?), and advertise your business in the proper section of this site, and see what happens. I'd be prepared to "give" away cases to members here to get word out, or maybe offer them cases at your cost for materials - just to get them in the hands of people here. Then once people are familiar with them, and can post reviews of your stuff, then you can start charging "full price" and build your business more.
Look at business models of other successful small companies and see how they've done it too.
Just remember, if you're doing the more custom high end route, then remember the economy still kinda sucks, and people aren't spending like they used to.