I've thought for a while that password managers would be ideal places to store WebAuthn private keys.
I already use a password manager to store my passwords so, as passwords move to WebAuthn, I'd like to use it to store my WebAuthn private keys as well, probably in parallel with a hardware security key like YubiKey.
WebAuthn supports roaming authenticators, which I believe could include a cloud service like a password manager. This idea seems obvious so I'm surprised it hasn't gotten any traction. Maybe there's an issue that I'm not aware of, and one of these days I need to do some deeper research.
1Password just announced support for SSH keys: SSH and Git, meet 1Password 🥰 | 1Password. Hopefully this is a step on the path.
Update 2022-04-01: For a very relevant proposal, see the "Copyable, multi-device Passkeys" section here: What does the future hold for modern authentication? - Yubico