I just don't know where to look / who to ask about "what can I make you?"
Thoughts?
Keep a notepad to hand and every time you see something novel that you like that you could make on your machine, make a note of it your first thoughts about what you specifically like about it and how you'd like to make a version that is your own.
Make things for yourself that you want without much thought towards profit. Document and photo these things, and share them with people around you - in time someone will either want one, have their own tangent they'd like to investigate, or know someone who's in the same sphere.
A wee thing I'm doing at the moment with my laser-cutting, having hit a bit of a creative block earlier this year, is I've set myself a wee project to force myself to do an iteration every day or so. Doesn't need to take long - I'm budgeting half an hour or so to design then cut and treat at the end of each day, and I'm already staring at a couple of pieces now that I'm really happy with and wouldnt've bothered coming up with otherwise.
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Also, short answer - signage :) A good 2.5D domain to practice in and make things that 'everyone wants'. particularly good in terms of finishing too, as there are sanding painting and varnishing challenges to be met too (where wood or MDF, etc, is concerned).
Good luck and, most of all - have fun!
Asking because a lot of people new to CNC (as you question seems to indicate) build their machine and don't know they have to spend a fair amount of time fine tuning to get the accuracy to where they want it to be.
Not sure if that's the case for you, but pointing it out, just in case. :)
With the 'metals' thought, be careful there. Soft metals (eg most aluminiums) are a lot easier to mill than the harder ones (eg cold rolled steel). It might be better to stick to soft metals for a while unless you're sure. :)