I think it would be a good idea to offer audio grammar lessons. They could include series[es] of lessons dealing with the different parts of speech, such as:
-Nouns
-Verbs
-Adjectives
-Adverbs
-Particles
-Sentence Structures
And stuff like that. However, by only being a free member, I'm not sure that something like that isn't already offered. (>.<) Forgive my ignorance, and someone give me your opinion on this.