Get the current entry/template fieldtypes in a plug-in

Question

I'm working on my first, more complex add-on and had a question on gathering field properties.

If I create a tag pair, which wraps a field inside a template, what would it take to get the tag properties of that fieldtype?

In other words, in the template I would have:

{exp:my_tag}{custom_wygwam_field}{/exp:my_tag}

Then in my pi.my_tag.php file, I would have something like this:

class My_tag {

public $return_data;

/**
 * Constructor
 */
public function __construct()
{
    $this->EE =& get_instance();

    $tag_content = $this->EE->TMPL->tagdata;

    // Gather the tag/field properties here...

    // Make whatever changes here...

    // Return the new content
    $this->return_data = $new_tag_content;
}