Browsing eww through tor.

I suppose, but it seems rather niche.

When writing into forms, automatically open a temporary buffer to prevent keylogging.

Pointless. A keylogger is most likely to run at the OS level, so it doesn’t matter what program you type into.

A copy wrapper that destroys the kill ring after yanking. (For sensitive data, like passwords.)

Generally not needed. auth handles storing many credentials without needing to use the kill ring at all. password-store has password-store-copy, which wipes out the kill-ring contents after 45 seconds.

In general, Emacs is not the right level to solve any of the problems you seem to care about; the OS is. You’d be better served by starting with a hardened Linux, like Skulls or Qubes, and customizing from there.