Some words are just more difficult to write than others. To know which words these are, a history of miswritten words would be helpfull so that this words can be trained especially
Practice your worst words
Words that are frequently typed incorrectly or slowly should be recorded, and practice sessions should be available to type these words to improve on our weakest skills. This feature would drastically help typists improve their WPM.
Give a feedback on the worst combinations of letters, and potentially practice them (e.g. all the words the have "qu" in them).
As opposed to practicing specific word, that you have trouble with, (which is like a symptom of a problem), this will attack the problem itself, (e.g. such as difficulty in typing 'x' after 'e' due to the way fingers work).