1. If the TOGA function is inhibited due to A/C being on ground then get an automated voice to call out "MANUAL THRUST REQUIRED" or some other warning.
If any button is inhibited, you should always get a buzzer sound or something for the time you push it. Humans want to relate reaction to action. If you push a button, you expect something to happen. If it does not, you will by mistake assume it happens. Only if something different happens (in that case the buzzer) you start using your brain because you are kicked out of your trained routine.
A completely dead button is one of the least useful things on an aircraft.