For Email Threads, we do have an option under the Sequence Settings which says 'Send follow-up emails as a reply in the same thread' (see the below screenshot).
If you turn 'ON' the toggle highlighted in the above screenshot, the system will still require a subject line for all email templates, however, if the template is being used in a sequence that is sending in the same thread, it will always assume the subject line of the email that was sent first in the thread.
The email threading will occur as shown in the screenshot below, which is a string of emails in a sequence, set to send in the same thread from Uptics.
Note: this is the view in GMail, and it might appear differently inside other Email Client providers.
The email keeps moving to the top of the inbox each time a new email is sent out, and it has the number of emails to the right to denote there have been, for instance, 3 emails in this thread. When you choose to send emails in the same thread, each email will assume the email subject from the initial email sent in the thread.