How about a front wheel mounted drivetrain with the motors built into the wheel hubs?
That would effectively add 4wd to old cars and allow the onboard engine to work normally with an option for eboost/edrive with the only extreme modification being adding the battery and control modules.
You would have to provide custom replacement hubs, for all the vehicle types you want to support. Plus, adding a clutch system to a hub drive is not feasable. So, the hub engines are engaged all the time.
I've considered it as a modification for FWD track performance cars. It is much easier to add a hub into a stiff rear axle. Turns out, that adding an electric motor, which is not part of the main engine assembly, ads way too much complexity in controlling the whole thing and weight to power ratio, that it stops quickly stops making sense.