What do you want to accomplish?

If your motor is running hot, then rewinding (with the same number of turns and thicker wire) might just help. Then again, it might not. Your losses are probably dominated by eddy currents in the core and poor connectivity in the magnetic circuit, in which case lowering resistance won't make much difference.

If you want a higher Kv constant for a faster no load speed, you can rewind with fewer turns. Adding turns will increase torque. But there's no free lunch. If you rewire a motor to pull more power at low voltage, current drain will increase, and you'll have to be careful to avoid reducing battery life.

Use multiple windings if you can't get the desired density with a single conductor, or try your luck with square cross section wire to get the same effect. There's no magic to multiple windings - it all boils down to the wire's total cross sectional area and the number of turns. You may get better density with multiple windings, especially when the wire starts to get thick (say >22 gauge for small motors - I've never worked on hub motors). Be sure to make your windings neat and tight: Wires that vibrate in use waste energy.