After some discussion in https://tracker.moodle.org/browse/MDL-7315 I have created a Specification located at: http://docs.moodle.org/dev/Moodle_Activity_Extensions_Spec
This document should be considered to be in draft mode.
This should be read in-conjunction with the forum thread at https://moodle.org/mod/forum/discuss.php?d=216784 which covers in some detail the specific functions that this module will provide.
I have created a specific forum thread at https://moodle.org/mod/forum/discuss.php?d=219620 for discussion of the spec and implementation.
We have completed about 60% of the Extensions module, and would like to contribute this to Moodle.