Hello
There is a risk involved in any custom instrument as you don't have the luxury of playing your exact dream in advance. Of course, there is incredible reward as well if you and your builder manage to nail it.
My best advice is to hit one of the well-known LARGE music stores in the nearest big city near you - and play a number of unique custom and boutique basses. It's expensive to visit the luthiers - but eventually you may need to. I like to pick out my own wood and it certainly makes a huge difference to be there in person.
One of the biggest issues for me is the how the body shape looks and feels both standing and on your lap. Also take note of all the custom features which appeal to you - LED's in the fingerboard, drop D tuner, number of strings, fretted or fretless, type of wood. Make sure you nail the neck feel, and measure it precisely - I find that duplicating your dream neck is one thing that nearly EVERY skilled luthier will do. Keith Roscoe duplicated my PRS #11 neck perfectly, and then duplicated it again, just widening the neck at the nut. I look at one as a Jazz neck and the other a Precision.
Once you find a body shape that feels great, then you can start to scour the custom builders. Maybe you will get lucky and find a standard body shape that a builder has cnc computerized routing on and won't cost an upcharge. Maybe that doesn't matter to you.
I have had incredible luck with Paul Reed Smith, Keith Roscoe, Rob Allen and I currently have projects with Patrick Cummings of Brian Moore/iguitar and Pat Wilkins. I am visiting Jens Ritter in a few weeks in Germany and I will see how that goes. All are amazing, some willing to step outside the box of the "offerings" they have labored over for years - and some not so much.
Good luck with it, it can be an incredibly rewarding experience if you take your time, put in lots of thought and advance work and eventually get it right!