Add a priority to push your function to the end of the hook queue.
add_action('the_content', 'my_plugin_content_hook', 1000);
Then, in your get_special_content
function you will need to apply wpautop
manually to the content to which you want it applied.
function get_special_content()
{
$text="";
$autopeed = 'content to autop';
$text .= apply_filters('wpautop',$autopeed);
$text .= "your textarea code";
return $text;
}
I believe that will solve your problem.