I would do 2 different ones-- a classic 4-string for blues, hard rock, punk, grunge, and speed metal-- and a modern, versatile 5-string for variety, metal, and session work.
The 4-string would be a G&L, sort of a "hybrid" between the "El Wunkay," "El Toro," and "El Tookay." The middle pickup would be a big Magnetic Field humbucker, paired with a soap-bar mini-humbucker a la "El Toro" bass in the bridge position. The switching would be pretty simple-- just a series/parallel switch for the big MFD humbucker and an on/off switch for the 18V active electronics, which involve a bass, treble, and a "tone" control that boosts lower-mids at one end, and "scoops" the mids at the other end. If the batteries die, just switch the active electronics off, and pull up on the active "tone" control to reveal a standard passive treble roll-off. Two volume controls (not a "blend" knob) round out the setup.
The body would be made of sand-blasted Swamp Ash, finished with a black pore filler and brightly-colored stains, similar to the 2-tone ash finishes by Sandberg and F-bass. The hardware would be chrome, with a black-crackle-finished bass bridge like the G&L's from the early 80's.
The 5-string would be a hypothetical "California Carved Top" bass by Carvin. The body shape would be loosely-based on the ICON body shape (which I think is gorgeous), slightly-modified (likely a bit more wide and stout) to accommodate a 35-&-1/4" scale, 24-fret neck. The neck contour would be wide-spaced asymmetrical, like the BB75, but with a wider nut, like ~1.9". It would have an alder back and a carved maple top with natural-masked binding, available in Carvin's wide array of translucent colors, although I'd order mine with a satin top coat. My personal instrument would also feature a jet-black ebony fretboard with white mother-of-pearl "Cemetary Cross" inlays.
Electronics would consist of the radius-top single-coil soapbars used on the Brian Bromberg model, with 2 volumes, stacked bass/treble, and stacked mid/sweep.
Fantasizing is fun, but it would be pretty awesome if some of our ideas actually sparked some creative juices with the makers/manufacturers we have been mentioning (in my case, G&L and Carvin)...