I've been given the task that when a customer with a certain property registers itself it should get a different e-mail template. What this property is, is not important for the question, but lets say for sake of simplicity I have a Service Contract that can check if the customer qualifies for the different e-mail template or not:
$this->customerChecker->hasCertainProperty($customerId); // returns true or false
Now this might seem like an easy task right? Just hook into `Magento\Customer\Model\EmailNotification` with a plugin or something, and modify the part where the correct e-mail template is selected. Right?