We need standards so that mid-sized companies can make parts that meet standards and other mid-sized companies can assemble those parts into cars.

The battery bus should be 2 wires (+ and -) and automotive-grade Cat 6. The voltage on the 2 wires should vary between 200 V DC and 400 V DC as the batteries are charged and drained. Any kind of battery can be connected in parallel. You can connect NiCad, NiMH, Li-Ion, LiPo, and AGM batteries all in parallel provided you put the right number of cells in each battery module to get the correct voltage. You can not put 2 different chemistries of batteries in series. So everything on the battery bus should be connected in parallel. The Cat 6 should run a variant of CANbus tunneled through Ethernet. If a battery can not handle the voltage on the battery bus (for example because the bus is at 200 V, and the battery is charged to 400 V) then the battery should disconnect. This is easily accomplished with small DC motors driving screws, just like low-battery cut-offs do.

Battery bays should be standard sizes. 18" x 30" x 6" would be good. That can fit under seats, under a truck's bed, and under a trunk or frunk. They can even fit under floorboards.

Motors should be housed in modular, sealed units with gearboxes and motor controllers. There should be spaces to install motors on every axle or even on every wheel. The gearing inside the motor units should give an output that is about the speed we expect from a car tire going down the road.

All batteries and motors should connect to the some parallel battery bus. If you want more range, add another battery. If you want AWD, add another motor or two.

All drive components should be connected by CANbus. Replacing your accelerator pedal should be as easy as a few screws and an electrical connector.

Brakes and steering still need mechanical linkages to comply with safety laws, for now.

A standard battery bus would enable other cool things:
1. Any car could have a generator to make it a 2-stage hybrid.
2. RVs could use the same batteries and generators as electric cars.
3. Off-grid applications and grid-tie applications could use the same batteries as electric cars. This makes it even easier to tie car batteries into the power grid.
4. Because battery busses and battery bays are standard, a medium-sized company can build capacitor banks that drop in.
5. Power transfers between 2 batteries on the bus can work, coordinated by CANbus-over-Ethernet. If you are driving down a mountain, you can regeneratively brake into your capacitor bank, then slowly charge your batteries from that bank.
6. Grid-scale battery deployments can use the same batteries as cars. The marginal cost of making another 1 million batteries is a lot less if you already made 2 million of them.
7. Battery chargers that plug into wall power, power inverters, air compressors, and other devices can also be slotted into standard battery bays and interface into standard battery buses. This gives a lot more flexibility to a work truck than the built-in air compressor some brands like to include. Imagine a 2-stage diesel-electric medium-duty truck (that's the size of a UPS truck or a big Amazon delivery truck) that has a hydraulic compressor in a battery bay so it can lift or lower the truck and a lift gate on the rear end.
8. You could buy a cheap electric car with a single Li-Ion battery and a single motor on the front axle. Later, you could add 2 more batteries to almost triple the range and another motor on the rear axle. Not all motors have to deliver the same horsepower, so your little 85 horse-power commuter car could become a 300 horse-power AWD machine. More upgrades could turn it into a 700 horse-power muscle car. This would be the biggest selling point. Buying a stripped-down electric car would be an aspirational purchase. The buyer's goal would be to one day upgrade it to a long-range supercar. Cars could grow with buyers. A kid's first car could be upgrade to AWD when they realize it slides on snow and ice. After a wreck totals the car, the motors and batteries could be moved to a newer model.
9. Concerns over recycling batteries and motors go away if most cars use the same interfaces to the batteries and motors. Old electric cars can be parted out like old Chevys. There is never a reason for an old Chevy small-block V8 to go to waste; millions of cars can use them. Ditto standard electric batteries and motors.

SAE and IEEE should coordinate to establish standards, then we should be able to swap out drive train parts as easily as we swap out wheels and tires on our cars today.

I wonder if the free market will adopt such standards, or if it will take regulations to push big auto makers into standardizing battery buses, packs, and motors.