As far as a market to target I'm not really sure.
The reason people are asking; "what would you like to make?" Is something worth serious consideration.
Most new machine shops are started to support a business, not to be the business itself.
For example; Let's say you want to develop a method and process to repair some sort of otherwise disposable (and expensive) oil well equipment.
The process you develop will dictate what machinery is required and defines a clear direction in which you will go - all business decisions need to be weighed against what you are trying to achieve. If it does not support the mission, don't do it.
So, to continue with our example -our focus is 100% on the mission of repairing oil well widgets... What do we need to carry out the mission of your business? Let's say you will need; a space to work, a VMC, a tig welder, a cylindrical grinder, specific test equipment and basic hand tools.
^^^these would be mission critical items for the business. If a lathe isn't required to get the job done, then a lathe should not occupy the work space until it is required.
Who is our target customer? Oil well equipment service contractors? drillers? Purchasing agents? Gas station attendants? Librarians?
When we pinpoint exactly who our customers are; we can find out where they are, study them and develop a custom tailored plan to market the product.
The owner's focus should always be on what the business sets out to accomplish. The machine shop is just a large tool box; custom built to carry out the mission of the business.
Many job shops that you see these days started out this way. They either lost their market due to competition, their product was phased out, or something else happened that pulled them away from what built the shop was built to do in the first place.
They are left as a business with nothing to make so they branch out and take on work in multiple directions.
The result is not always bad. Like @Garwood said; You can never tell where fate will lead - for some businesses they will adopt a new, far more lucrative niche and this is when the real growth starts to happen.
TLDR: there are many shops, already well established who can and will make parts to print. You have to come up with a mission; plan and goals so that you can make and market your specific products or services. Without doing this first, you will likely waste time and money figuring it out.