CB

This should just be an extension of the existing Multi-Select fieldtype except it takes unknown stored values and converts them to options specifically for the user that has them stored. Example as follows (as stored in database).

Option_1|*|Tag_1

Option_1 exists as part of the field while Tag_1 was added by the user. Tag_1 would only be available to the user. As for the tag input it self it's easily done using cbselect.