I find the prospect of creating my own custom bass daunting, having already done it once and ended up with a bit of a lemon which I progressively modified into... ...a modified lemon.
One bass would have to be all things to me and my tastes range from Jaydee to Smith to Musicman. Clearly an unreasonable request, so I haven't bothered. In terms of the sound in my head, only one bass has hit the spot (and its not a Smith) but the maker has told me he's not interested in making custom instruments unless he's commissioned on an hourly rate.
I think I'd be more comfortable tweaking an existing design rather than starting from scratch.