I was just thinking that a homophones unit would be a good idea? Because it's hard to tell the tones apart, so maybe having a group of questions composed of just telling the difference between words that sound the same apart from the tone and then having to select the correct word would help us learn the differences between each tone. For example, 师，时，十，事 could be the options and we have to pick the correct one from an audio track.
I agree, but it would be too hard near the beginning of the tree. ma is a word that has many different tones and different symbols for each. It is used on other sites to demonstrate the difference in sound. Seriously, google “Chinese tone ma” and you will have your pick of them.