It would be nice if there was a Kasa smart switch (relay) available for the control of existing lighting, say like a ceiling light. This would work like your HS103 except instead of an AC plug input/output it would take 120VAC in (Black/White wires in) and switch the 120VAC (Black/White wires out) when turned on. It would need to be small enough to mount in the existing wall switch box and work with the existing wiring in that box, essentially replacing the old, mechanical wall switch. Then a dummy plate or cover could be mounted to the electrical box to replace the existing switch wall plate. And then a Smart WiFi switch could be mounted to the dummy plate with adhesive to replace the old mechanical switch.
This would allow old, existing hardware to be integrated with and controlled by Kasa hardware.
Are there any plans for such a device?