My proposal on this issue is that group of experts develop techniques and the use of scientific technology that can examine cards and judge if they have been altered. There are very sophisticated techniques used in other areas (art, antique artifacts) but not used in the sports hobby.
I envision the examination being done on expensive cards. At some point, enough expansive cards will have been examined that the onus will be on those who choose not to have theirs examined.
Documentation of provenance will be required.
I would prefer it not to be another $$-making business, as that obviously corrupts things, but more of an academic, non-profit entity. But that's lesser of an issue, as the development and use of the technology is what is important. And if grading companies wish to adopt the techniques that is fine.
As someone mentioned, it would be more of a scientific study giving reports, rather than a 'purple sticker' opinion service.
If you want more background on scientific methods used in different areas, see the following
book.