I have a few assumptions to make before trying to answer your question:
1. A custom knife will be ergonomically superior over any other knife for me
2. It will have unique features that serve a purpose. An example of this would be the Emerson Wave or the HRD
3. It will cut better than my other knives
4. The mechanism will be exactly the way I design it to be
5. The knife should be attractive
Now, onto the things that would help get me into customs, based upon these assumptions.
1. For ergonomic superiority, I want to be able to meet with the maker. I would want measurements taken from my hands, and I would want him to take note of the way I use my knives and work with me to design the curves of the knife to fit me flawlessly. I would likely want a flipper, and I have a very specific set of wants for the flipper protrusion to be perfect for me.
2. As far as unique features, I would want to work with the maker to make the action of the knife better than perfect. I want to do engineering analysis of the design to make sure that every force is balanced flawlessly, and I want every piece of the knife designed to contribute to the robustness of the balance. I want to use a unique bearing system, I wouldn't want the pivot designed around available caged bearings.
3. The cutting performance should be easy for any custom makers. My tasks are very light, so I would just need a knife in a good steel with a fairly thin edge.
4. This is the biggest problem. If I want a custom knife, it will be everything exactly how I want a knife. That means no liner locks or frame locks, no back locks, not even an axis lock, my lock design will fit a very specific set of needs (more wants than needs, really

).
5. Similar to 3, pretty much any good maker can make a knife attractive. After all, beauty is in the eye of the beholder, so if I work on the design I will probably find the knife attractive
I don't need the knife to be handmade, in fact I would be quite satisfied with many CNC parts, perhaps just with hand finishing to ensure excellent fit. The biggest problem is that I have no reason to buy a custom knife that isn't perfect for me, and I've never seen anyone make a knife that is even remotely close to my kind of perfect.