It seems like one clear way that Honda could improve the IMA system while still keeping a single drive-shaft is to have a clutch positioned between the IMA and a electric motor that could disconnect the engine when crusing around at slow speeds or coasting, so that it would not be a source of friction/resistance. For coasting down hills, that clutch could be released to allow the regen alone to control the speed of the car, and collect a lot more energy in the process without wasting it with engine friction. If the IMA alone were not enough to control speed safely (such as the battery being full), the engine coudl be re-engaged automatically.

While this type of EV mode woudl not be as powerful as the prius, it would allow a driver in stop-and-go traffic to roll along at slow speeds.

Another thing would be to allow more control, such as a way to manually force an "auto-stop" without using the ignition to restart. Another thing would be the ability to adjust the aggressiveness of the assist logic, such as favoring heavy assist (for a short, steep incline coming up) or disabling all but minimal assist (long gradual climb). There could be a few "modes" that the driver could select for those different scenarios. A cruise-control that woudl automatically lock in a certain gas throttle level, then vary assist/regen alone to maintain steady speed would also help FE.