What is a privacy friendly application that you'd love to have, but no one has developed yet?

KeePass with inputstick. It's a device that plugs into a USB A port, and your phone talks to it via Bluetooth. It emulates a keyboard (and mouse if you want), and there's a KeePass plugin for KeePass2Android.

You open one of your password entries, click the username, and it types the username on your computer via inputstick. Ditto for passwords and totp or other fields.

You can also use inputstick to just remotely control your computer, albeit locally only and without a monitor connection. I've used it to control my raspberry pi or android TV, aside from password entry.

With this, you can have your password database be completely offline and your computer have no lasting knowledge of your passwords. Of course, a keylogger would still get the passwords that are "typed".

I've had one of these $40 devices for a few years. I don't use it too often, as I tend to synchronize my KeePass database on all of them, but it does come in handy. I wish the developer of the hardware made a usb-c one, but it works with usb-c to usb-a dongles.