Disable formatting on textarea

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.