We all know that there are 4 different skills to master in learning a language:  understanding; speaking; reading; and writing.  It occurs to me that there must be some way to evaluate a person's language ability on each of these measures and then use an appropriate mix of methods to improve on the person's weaknesses.
For example, my boys, for their ages, have a good level of French comprehension, and as a result of this and some practice, a good level of reading ability.  Their speaking ability however is less accomplished and (again partly as a result) they struggle to write in French.  On this analysis, it should be possible to suggest a teaching method which incorporates perhaps a larger amount of conversation practice, but still reinforces the comprehension, reading and writing.  Such a guide might suggest methods that build on their more advanced reading and comprehension skills to bring along their conversation and writing skills. 
If such a manual doesn't exist, does anyone want to write one? :-)