Total mail UI/UX overhaul, total backend overhaul allowing implementation of new protocols (first new protocol was Exchange), an Android app and monthly release cadence vs yearly. And that's just to name the ones that immediately come to mind. Have plenty of users yelling at us for changing too much.
Unlimited aliases at custom domains are a part of the offering. Technically, Thundermail supports sieve rules, we do need to come up with UX to expose it to users for management.
No, we rely on PGP encrypted email. What I'm talking about is the latter. Emails are encrypted with your public key before being stored in the database. (Our stack is based on Stalwart, you can read about how it works here: https://stalw.art/docs/encryption/overview )