Came here in search of the exact same feature.
Having to manually select the email template every time our team sends ticket updates is chipping away at our productivity!
We really need the ability to set a default email template for each ticket pipeline.
This feature would be even more powerful in tandem with this other feature request regarding setting default properties for each ticket pipeline: https://community.hubspot.com/t5/HubSpot-Ideas/Ticket-Templates/idi-p/246063