Thank you for your feedback, we've added your feedback and your use case to our feature tracking system. Tracking state change history had been requested before in the past, but custom field change history had not, so I added this. At this moment in time we don't have an estimated release date for this feature yet though.
For now it is already possible to see who made manual changes to an asset though, without seeing the specifics of the change. This information is stored in tblConfigLog. More on this functionality can be found
here.