Magento – Magento2 move coupon code inside summary

Yes,it is possible.

• Create A cart rules from admin>Marketing>Cart Rules.
• Create a rules which will create dynamic coupon by selecting 'Use Auto Generation' for create multiple couple coupons.

During Creation setting should be :

Now,Add custom data in order email template in Magento 2 and create a coupon of the rule,you should fire run observer at event email_order_set_template_vars_before.

$this->eventManager->dispatch( 'email_order_set_template_vars_before', ['sender' => $this, 'transport' => $transport] );

So, at this event you can add new parameter via transport to template means you can create coupon and send to email.

Just like: events.xml

<?xml version="1.0" encoding="utf-8"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="email_order_set_template_vars_before">
        <observer name="add_Custom_variable_to_Order" 
            instance="[Vendor]\[ModuleName]\Observer\ObserverforAddCustomVariable" />
    </event>
</config>

And at observer will create coupon and send to to email:

<?php
namespace [Vendor]\[ModuleName]\Observer;
use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\App\Request\DataPersistorInterface;

class ObserverforAddCustomVariable implements ObserverInterface
{

    protected $ruleFactory;
    protected $massgenerator;
    protected $logger;

    public function __construct(\Magento\SalesRule\Model\RuleFactory $ruleFactory,\Magento\SalesRule\Model\Coupon\Massgenerator $massgenerator, \Psr\Log\LoggerInterface $logger ) {
        $this->ruleFactory= $ruleFactory;
        $this->massgenerator = $massgenerator;
        $this->logger = $logger;
    }

    /**
     *
     * @param \Magento\Framework\Event\Observer $observer
     * @return void
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        /** @var \Magento\Framework\App\Action\Action $controller */
        $transport = $observer->getTransport();
        $couponCode = $this->createOneCoupon();
        if($couponCode){
            $transport['free-coupon'] = $couponCode;
        }
    }
    protected function createOneCoupon()
    {
        $ruleModel = $this->ruleFactory->create();
        $ruleModel->load({RulesID});
         try {
                $data = array(
                        'rule_id' => 1,
                        'qty' => 1,
                        'length' => '12',
                        'format' => 'alphanum',
                        'prefix' => 'free-shipping',
                        'suffix' => '',
                        'dash'=>0
                        );


                $generator = $this->massgenerator;
                if (!$generator->validateData($data)) {
                   return false;
                } else {
                    $generator->setData($data);
                    $generator->generatePool();
                    $generated = $generator->getGeneratedCount();
                    $codes = $generator->getGeneratedCodes();
                    return $codes[0];

                }
            } catch (\Magento\Framework\Exception\LocalizedException $e) {
                $this->logger->critical($e);
                return false;
            } catch (\Exception $e) {

                $this->logger->critical($e);
                return false;
            }
    }

}

At the email template , you can get this custom variables free-coupon using {{var free-coupon|raw}}.

how to add custom data in order email in magento 2

I haven't tested it, but perhaps you can do something similar to vendor/magento/module-catalog/Block/Product/ProductList/Crosssell.php _prepareData() function:

public function getItems()
{
   $product = $this->_coreRegistry->registry('product');
    /* @var $product \Magento\Catalog\Model\Product */

    return $product->getCrossSellProductCollection()->addAttributeToSelect(
        $this->_catalogConfig->getProductAttributes()
    )->setPositionOrder()->addStoreFilter();
}