Template filter for custom taxonomy terms

I think you’re looking for this dynamic filter within the get_query_template() function:

    /**
     * Filter the path of the queried template by type.
     *
     * The dynamic portion of the hook name, `$type`, refers to the filename -- minus the file
     * extension and any non-alphanumeric characters delimiting words -- of the file to load.
     * This hook also applies to various types of files loaded as part of the Template Hierarchy.
     *
     * Possible values for `$type` include: 'index', '404', 'archive', 'author', 'category', 'tag', 'taxonomy', 'date',
     * 'home', 'front_page', 'page', 'paged', 'search', 'single', 'singular', and 'attachment'.
     *
     * @since 1.5.0
     *
     * @param string $template Path to the template. See locate_template().
     */
    return apply_filters( "{$type}_template", $template );

where in your case the type is taxonomy.

You mentioned your own custom plugin. So if you want to override the template for your hello-cat term of your custom taxonomy, with a custom template within your plugin, then you can try the following within your root plugin file:

add_filter( 'taxonomy_template', function( $template )
{
    $mytemplate = __DIR__ . '/templates/custom.php';

    if( is_tax( 'some-custom-tax-slug', 'hello-cat' ) && is_readable( $mytemplate ) )
        $template =  $mytemplate;

    return $template;
} );

where the custom template is placed within your custom plugin folder, like:

/wp-content/plugins/my-plugin/templates/custom.php