we have recently developped eth-walletconnect-keyring, an implementation of MetaMask's Keyring interface that uses the WalletConnect protocol to connect a (hardware) wallet to Metamask. A beta release of Metamask with this solution is available here for testing:
https://github.com/Toporin/metamask-extension/releases/tag/v10.13.0-WC

This is an early proof-of-concept, but it would allow any wallet (hardware/software) that supports the WalletConnect protocol to connect to Metamask. It is a standard and neutral solution, quite simple and without USB/hardware requirements, so we hope that the Metamask team would be more willing to integrate it (like they did with the QR-code approach supported by multiple vendors).

Here is a short demo showing how to connect a wallet to Metamask through the eth-wallet-keyring plugin. When using the WalletConnect option, a new tab opens with a react app that will handle the actual WalletConect connection. In this demo, we use a local server (for development only), in production, the react app is currently hosted on github-page.

Would you be interested to check this approach, and maybe collaborate with us to push this solution forward? The more people involved, the greater the chance to have a pull request accepted!

Everything is open source, questions, comments and pull requests are welcome!