This is already what is happening with SafetyNet on Android. For now most applications don't require hardware attestation so you can pass by spoofing an old device that didn't support hardware attestation but I'm sure that will change within a decade.
Being able to trust the security of a client can protect against many attacks and it is up to web sites to evaluate what to do with into information that a client is proven to be secure.
- What is the least expensive device that can be certified like that? The least expensive process?
- What is the highest level of openness such a device can offer to the user, and why?
To my mind, it would be best to have an option of a completely locked down and certified hardware token, a device like a Yubikey, that could talk to my laptop, desktop, phone, or any other computing device using a standard protocol. As long as it's unforgeable, the rest of the system can be much. much less secure, without compromising the overall security.
I don't know. I haven't personally gone through the process.
>What is the highest level of openness such a device can offer to the user, and why?
You have to follow the CDD. https://source.android.com/docs/compatibility/13/android-13-...
and you of course must pass the compatibility tests. So it can be as open as you would like as long as you do not break the android security model.
>it would be best to have an option of a completely locked down and certified hardware token, a device like a Yubikey
That approach is limiting since secrets can't be passed to the host operating system and compute with secrets have to happen on the secure device.
AKA as long as you don't give control to the user.