5 Ways to Make your Plugin Really Extensible | http://ibenic.com/5-ways-make-plugin-extensible

add_theme.php

<?php

add_theme_support( 'my-plugin-style' );

edd_buy.php

<?php

// function edd_complete_purchase
do_action( 'edd_complete_purchase', $payment_id );

edd_cart.php

<?php

// class EDD_Cart
// method get_item_price
return apply_filters( 'edd_cart_item_price', $price, $download_id, $options );

// Only the returning parameter is passed
add_filter( 'edd_cart_item_price', 'change_item_price' );

function change_item_price( $price ) {
  // Do something with the price
  
  // Always return!
  return $price;
}

// 2 Parameters
add_filter( 'edd_cart_item_price', 'change_item_price', 20, 2 );
function change_item_price( $price, $download_id ) {}

// 3 Parameters
add_filter( 'edd_cart_item_price', 'change_item_price', 20, 3 );
function change_item_price( $price, $download_id, $options ) {}

shortcode.php

<?php

function my_plugin_shortcode( $atts ) {
	$atts = shortcode_atts(
		array(
			'foo' => 'no foo',
			'bar' => 'default bar',
		), 
    		$atts, 
    		'my_plugin_shortcode' ); // Third Parameter makes a Plugin Extensible

   	return $output;
}
add_shortcode( 'my_plugin', 'my_plugin_shortcode' );

// Extending
apply_filter( 'shortcode_atts_my_plugin_shortcode', 'extending_shortcode_atts' );

function extending_shortcode_atts( $atts ) {
	$atts['foo'] = 'Yes Foo';
	
	return $atts;
}

shortcode2.php

<?php

function my_plugin_shortcode( $atts ) {
	$atts = shortcode_atts(
		array(
			'foo' => 'no foo',
			'bar' => 'default bar',
		), 
    		$atts, 
    		'my_plugin_shortcode' ); // Third Parameter makes a Plugin Extensible
    $output = '';  
    ob_start();
    
    ?>
    <strong>My Awesome Shortcode</strong>
    <?php
     do_action( 'my_plugin_shortcode_after_title', $atts ); // Adding an action hook and passing attributes
    ?>
     <p>Some content</p>
    <?php
     do_action( 'my_plugin_shortcode_end', $atts );
    $output = ob_get_clean();
    // Enable Filtering
   	return apply_filters( 'my_plugin_shortcode', $output, $atts );
}
add_shortcode( 'my_plugin', 'my_plugin_shortcode' );
 

template.php

<?php

add_filter( 'template_include', 'my_plugin_templating' );

function my_plugin_templating( $template ) {
  $file = false;
  
  if( is_singular( 'my-cpt' ) ) {
    $file = 'single-my-cpt.php';
  } 
    
  if( is_page( $page_id_from_our_settings ) ) {
    $file = 'page-my-page.php';
  }
  
  // There is a file, create a new template path
  if( $file ) {
    // Check in Theme for folder my-plugin/templates/ and the file inside
    $template = locate_template( 'my-plugin/templates/' . $file );
    
    // There is no template in the theme
    if( ! $template ) {
      // Get from Plugin's folder 'templates'
      $template = untrailingslashit( plugin_dir_path( __FILE__ ) ) . '/templates/' . $file;
    }
  }
  
  return $template;
}

theme_support.php

<?php

add_action( 'wp_enqueue_scripts', 'your_plugin_scripts' );
function your_plugin_scripts() {
  
  // If the current theme supports it, don't enqueue it
  if( ! current_theme_supports( 'my-plugin-style' ) ) {
    wp_enqueue_style( 'my-plugin-css', plugins_url( 'assets/plugin.css', __FILE__ )  );
  }
  
}