The brake disk its self could be the rotor in an axial topology AC induction motor. No magnets needed.
You could still have your brake pads and caliper for hydraulic brakes.
Then the rest of the circle is taken up with the stator coils.
I'm sure this has occurred to many of us while daydreaming about future possibilities, but I usually imagine the rotor as PM. Induction helps both cost and temperature tolerance (needed because of the friction brake), but adds a challenge for regenerative braking at very low speed (because there is no torque without slip).
The rotor ends up completely encircled, which is a serious cooling challenge, so it would need to be vented (which is normal for front brake rotors, but not rears on production cars), with airflow radially through the stator.
Windings would normally be a major issue for a rotor which needs to withstand being used as a friction surface for braking; however, the intention would presumably to use the normal squirrel-cage design (rather than the relatively rare wound induction rotor); due to the axial flux the conductors would run nearly radially and be shorted by rings at the rim and inside of the stator face area. To my surprise - although in hindsight it makes sense - these rotors do not have insulation between the conductors and the laminations, so the brake pads could actually slide across the conductors and wear down their surface. Still, I wonder about the compatibility of a mix of iron and whatever the conductor material might be (aluminum or copper).
It may even be practical to use a solid aluminum rotor, although this would presumably be less efficient than a design with conductors across iron laminations, and would be problematic as a brake rotor. Aluminum brake rotors do exist - they're even a regular production item from Wilwood
- but they are not ideal (for multiple reasons); they require an anodized, ceramic, or plasma-coated surface, and I don't know how that would affect induction motor performance. Solid aluminum would be simple.
There are two major issues with the motor-instead-of-brake idea: the loss of the brake, and the effectiveness of the motor. WolfTronix's dual-purpose rotor idea addresses the first, but still leaves the second.
If you could manage 5HP continuous per wheel, and perhaps 10-25HP peak, it could work as a bolt on hybrid conversion.
Even if the brake issues are manageable, as a motor the interrupted stator is interesting. With a PM design I can see how this could work, but with induction it is not so clear. Even if three-quarters of a stator is three-quarters as effective as a full stator, this is still a motor running at wheel speed, and not an optimal one at that.
The combination implies (in practical terms) that the motor gap would be open, rather than enclosed.
It might be a "bolt on" conversion, but it would not be a simple one, because the vehicle will have a mounting bracket for a brake caliper... not for a stator assembly. Vehicles designed for drum brakes but available with disks would probably be easiest, because they would accommodate a stator mounting plate similar to a drum brake backing plate. It also looks like a workable stator would be bulky compared to a typical disk brake caliper, so fit may be a challenge.