I'm struggling to find a way to set a subject line in the email template, is this not possible? Ultimately, I will be pulling the subject line from a data extension with some ampscript (and to do that, I need to set the ampscript variable in the subject line in the template) but I'm not seeing a way to set any subject line at the template level. Is this really not possible?
The templates in Marketing Cloud doesn't include the subject definition meaning that you will always need to create an email to set-up the subject.
You can assign the subject text to a variable using ampscript in the template and then, once you create the email based on that template, you just have to put the variable in the subject field.
You may use setSubject method in Apex