Create a separate page in Wordpress that allows users to comment on a self-selected post using AJAX Autosuggest by tag. Should by SQL injection/XSS resistant.
Example:
Field a: Radioboxes: a) boats, b) trains, c) cars [correspond to wordpress tag boats, trains and cars]
If selected c) cars: display form field (text box) with autosuggesting posts.
Then show comment field, and then add comment to post selected above.
[In order to get this job, mention briefly how you will prevent XSS and SQL injection in Wordpress (tech terms)].
Note I am using my own theme- so this has to be coding-only, no install on server. Only respond if that is OK.