A measure of creativity from mind maps
You are not far off from one of the classic tests of creativity!  The Torrance Test of Creative Thinking scores for fluency (number of responses) as well as originality (statistical rarity of responses) and elaboration (level of detail) on a variety of different tasks--some of which are similar to your mind-mapping task.  The validity of the TTCT has been examined with several long-term studies, so there is a fair amount of data on its measurements as well as the form of the test itself.

Torrance, E. P. (1981b). Empirical validation of criterion-referenced indicators of creative ability through a longitudinal study. Creative Child and Adult Quarterly, 6, 136-140.