You could implement filters and actions in your core plugin’s code, at the appropriate places where you want your addons to interact, by using:
do_action( 'my_action');
apply_filters('my_filter', $value, $variable_to_pass, $another_variable_to_pass);
Then you can have addons as separate plugins and have them interact with the core via these hooks by calling:
add_action('my_action', 'action_handle_function');
add_filter('my_filter', 'filter_handle_function');
this is how plugins interact with the WordPress core so you are just following in those footsteps.