This paper presents the initial stage in the design of an ICALL system. The objective is to develop a system that automatically generates linguistic analytics of L2 learner writings. Student texts will be processed with NLP tools producing different types of textual measures. We present the design of a new functional complexity metric aiming to capture the paradigmatic competition between forms mapped to the same communicative function, i.e. microsystems. More precisely, we analyze the variations of the FOR and TO prepositions in terms of frequency and probability of occurrence. Relative frequency shows significant correlations with CEFR levels suggesting its possible use in an analytics report system. More work is required to extend the approach to other microsystems.