Luke, it's definitely possible to achieve what you want. Whatever is in your template that is not marked with a Marketo-specific tag (mktoText, mktoImg, etc.) will be baked into the template. If you change it, it will change in all emails that use the template. When you define a section as "editable", however, it now becomes controlled by the Marketo Email Editor (hence "editable" element). Templates don't override editable elements because the information architecture defines those as being email-level sections. The template will control the placeholder contents when a new email is created but there is no connection from that point forward. Marketers would edit their body text or images from that point for each individual template.
I also actually disagree with your comment about modules. The modules defined in the template truly are "template modules" which can, in turn, be added to emails. It is a 1:n relationship...you can add 20 modules of the same type into an email and structure them all completely differently with different contents, formatting, structure, spacing, colors, etc. We do not want to have a template update to a template module overwrite all of that work at the email level. The idea is, marketers may utilize several modules of the same type to build their emails in very custom ways. If you are truly trying to lock down the look-and-feel of the email, why use modules at all? Just build your template with no modules and you'll have full control to overwrite every email in Marketo.
Hmm, researching similar topic and I'm finding different experiences and opinions of the intended vs. actual outcome of updating an existing email 2.0 template and whether or not the updates carry through to existing, approved assets. And additionally whether it's best to approve or discard the drafts that are generated as a result of a template update. I have recently experienced something similar to what Tyson Jurgens described here: https://nation.marketo.com/community/champion/blog/2017/05/08/bulk-approve-landing-pages-in-the-desi... However, I found your reply above that states this should not be the case. Marketo support has advised that we create net new templates and archive the existing versions to limit any issues caused or draft states of existing email assets - but seems like this defeats purpose of limiting number of templates in your instance? Curious about additional thoughts here.
I took a look at that thread and couldn't immediately understand what the issue was there, but what I'm saying in this thread is definitely the expected behavior. Here's some tips + tricks:
If you are seeing behavior different from what I describe above, please let me know. Also, feel free to ping me if you have specific questions about something you're seeing.