The ability to add accounts from airgapped devices such as Parity Signer, Stylo, and Keystone hardware wallet and to support all wallet functions from these secure devices.
Parity signer is now supported, Keystone is planned in the roadmap!
+100, this is a blocker for me.
I'm not entering my seeds (unless they are proxy-accounts) in anything what touches online. Even if it is a secure open source extension of a team that I trust.