Hi Kristian,
It took many years for me to come to understand what I like and what I do not like about an instrument. As it is a combination of quite a few variables (body wood, neck wood, fingerboard wood, neck joint topology, metal components, nut, pickups, electronic circuit including a preamp, etc.) and each of these variables contributes to the sound and playability of the instrument to some extent, it requires time to figure out what you like and what you don’t.
To cut the long story short, I would not go the custom route at the beginning, but rather pick a generic instrument (like a Squire P or J, or a Jaguar) and see what the shortcomings are, if any, to your liking. By modifying that kind of an instrument, you can still achieve an amazing bass at the end. If you still like to go for the custom route, I would start with the simplest design (passive, one pickup, no control, etc.) and over time I would add options.
I see that you are based in Turkey, where I recently moved back to. If you would like, I would be more than happy to sit with you and chat over some options. Also talking to some local luthiers like Erkan Sizarlar (Izmir), Ozgur Turan (Eskisehir), Veli Boyraz (Balikesir), Briken Aliu (Istanbul) would help. Apart from Veli, I know the other three and I can hook you up with them.
Best,
Alper