add an option to play the newer characters more frequently when learning by the Koch method
I think this is vital. Just Learn Morse for Windows does it this way, and after you get about halfway through the character set (say about at 'g' or so) Ham Morse is way too slow to learn from. Hearing the new character less than five percent of the time is not enough to learn it. I find that I have to learn new characters using JLM. Ham Morse is relegated to practicing characters I already know while I rise the train.
...as an addition to this comment, maybe add the ability to increase the frequency of "troublesome" characters so that one can get a little more practice with them. For example, with the MFJ unit, you can add "G" three or four times as a selected character and get a little extra practice with it until you get it right.