It looks like you're trying to get a feature request implemented. You should totally drop that and try jQuery. ;)
I've written a script (instructions) that suggests tags for you! You can get suggestions based on any tags you've already supplied, or based on any tags that similar questions use. (Suggesting based on already supplied tags will only return tag suggestions that are related to all of the tags. It's pretty limited that way.)
The first method works by making an API call to find out what tags are related to yours.
The second method works by scanning the page for related questions. On the Ask page, these are the questions that pop up underneath your title. When editing tags, these are the questions in the "Related" sidebar. Once it has these questions, it makes an API call to find their tags, and displays the tags sorted according to which are most common among the related questions. (There's a special case for the
/posts/<id>/edit page. There it uses a different API call to find the posts related questions' tags.
It works when asking questions, when editing questions, and, for 10kers, even when retagging questions (yay!).
Fair warning, API calls are rate limited, so if you use this too many times a day (like 10,000) it may stop working. Mashing the Suggest links won't work though, there's some caching. Sorry. ;)