> i.e. the petrol engine effectively just runs a generator to the batteries. You don't need prop shafts/gearboxes/transfer case/axles/alternator/starter etc.
But you can't take advantage of that without handling every specific make of car. You need to design a specific solution for each engine.
> If you can integrate the motor assembly into the wheel hub you don't need any kind of axle at all.
This has a lot of downsides; the unsprung weight is very high which makes the ride much harsher. It's also very hard on the motors because of the shaking. There's only a few inches of space to claim in a normal wheel well by replacing and widening the tire. If you replace the suspension knuckle you have to make it work for all different makes.
All that said, I do think there's an underserved opportunity for a drop-in solution that just replaces the engine. Basically the same thing you're proposing, but why keep the old engine?
Replace it, and you can make a series-parallel hybrid, which is the ideal type. It's an engine connected by a clutch and single reducer to the driveshaft, with a motor connected to the engine (or by gears). At low throttle the motor acts like a generator, using the opportunity to increase engine load, charge the battery, and run the engine in a more efficient torque. At high throttle the motor joins the engine, increasing power. At cruise, the motor freewheels. For short trips the engine disconnects and the car runs on battery. You don't need an additional generator, the engine gets an efficient shaft connection, and it can still run battery-only. Best of all worlds and IMO worth trashing the engine.
If you drop in a full unit with batteries etc you might need a few configurations for different engine bays, but you can just have a variety of mounting points for different makes. Plug into the radiator for cooling. Give a serpentine belt wheel for accessories, or just have an AC onboard and use a spool valve to split some off to col the batteries better.