I would love if the Shopify theme editor had integrated git support.
Here is how I would imagine it to work:
when adding a theme you can select from Shopify default themes, uploading a custom theme (3rd party) or linking a repo (github or bitbucket).
When selecting a repo you can choose a branch (master, dev1, dev2). This means we can have master as the live version on the site and all changes that are pushed to the repo are replicated on the site as well as dev branches where we can preview our changes.
This means we can develop locally and view our changes on the go and then once we are finished developing a feature we can merge into the master branch
I'm sure I have missed something but I also think that this feature would be useful to many teams who use Shopify.