Make the Default sent from SharePoint Emails (like Quota limit and Access request) fully configurable in Central Admin
This would allow (without hacking at the default files) to set branding on the Emails in a HTML based format to make the emails more meaningful to users.
Also the Quota limit email should be more meaningful and a developed solution I have seen was to disable the timer job that actually runs with a custom version and then send out a mail with a more visual representation of the Storage being used ( Simple bar showing total amount of storage allocated to the site with the amount used and how much was in the recycle bin.)
Also I would like to see this being able to be applied to SPO as well to keep the end user experience consistant between on Prem and Online
I agree. Also, welcome mail after my site creation should be configurable...
Tom Burke commented
Additional related request: Make Alert emails and any other type of email configurable at the site level in the fashion mentioned above. A SharePoint HTML editor would be enough to style it. And, make this available to SPOnline users as well as on-premises!
This request is important for branding as well as minimizing confusion among end users, because often it isn't really clear where a SharePoint-generated email came from or why it was sent.