DANE (DNSSEC) would be hugely beneficial to the security and privacy of ProtonMail. DANE makes Man-in-the-Middle attacks practically impossible.

This is a necessary addition and should certainly be prioritized over-all, as it is at the core of what ProtonMail represents: privacy and security.

It's currently a feature available over at Tutanota (https://tutanota.com) and definitely sets it apart as an arguably more secure option, here's a great read on the topic: https://tutanota.com/blog/posts/dane-everywhere