I had someone access my Drive through an active session I was not present for, and I think it would be a very valuable tool to be able to protect individual files

I see just as much value for this feature even if one is not sharing their account with anyone.

Being able to control which files are decrypted on your device would be an excellent security feature. I would love to be able to keep most of my files easily accessible on my desktop while having the option to keep sensitive files encrypted until I need them. There are a number of tools to accomplish this (Cryptomator, Veracrypt, etc.), but it would be fantastic if Proton handled this natively.

I think OneDrive's personal vault would be the perfect target to imitate.

A few options related to this I'd like to see in addition to this is a pin to unlock the web tab, similar to the pin to unlock ProtonPass after a set time. Also, there should be an option for hidden folders, just like on Linux/Windows/iOS. An additional reasoning for all of these options is as a secondary security feature for users logged into Proton on a shared or work computer.

Something like the Personal Vault in Onedrive, right? I hope this gets added.

It will be nice to be able to add another password for some folders as an extra layer of security if one wishes

Please I would like to be able to create a secured password-protected folder in proton drive.