I think this might make sense, as I imagine most people prefer the standard PIN input in most cases, but there might be situations where you consider shoulder surfing a real threat, so you might want to scramble your PIN in that case.
It is not yet clear if we want to implement this, and what kind of complexity it could introduce, but I will likely file an issue on GitHub for this if we decide it's something we'd like to explore.
The current PIN scrambling option should remain, as some people might always want it to be scrambled and might not want the extra button press before entering their PIN.
It is not clear if the best UX would be for the "scramble" button to be present when PIN scrambling is enabled by default. It would make sense to be able to rescramble, but if you feel like you need to re-scramble, that means someone was close enough to be able to read the numbers off the screen, at which point PIN scrambling doesn't help you a lot.
There's also another school of thought here that says using a fingerprint when there's a risk of shoulder surfing makes the most sense.
GrapheneOS will also soon have 2FA for the secondary unlock method, meaning that it will (optionally) not just be a fingerprint, but a fingerprint + PIN, allowing you to make the primary unlock method a large passphrase etc.
The secondary method being a PIN + fingerprint defeats shoulder surfing (as fingerprint auth is required) while also making it more secure (as a PIN also needs to be known, someone can't just force your finger on the scanner and call it a day).
All in all, it's an interesting idea. I'll have to think about it more. :)