Templates to design Emails
Build in email design and prototyping features. XD already shines in its ability to quickly layout a single email design or campaign, but it really isn't designed to build emails. Yes, emails can already be built quickly and handed off to developers using XD. But, if XD would incorporate a few features geared to emails it would be a great and welcome asset. Features like; templated documents for common email sizes. Maybe templated headers for To:, From:, and Subject:. Maybe UI kits with email header elements.
I'm sure others can contribute more specific feature suggestions.