Suffixes mandatory for data but optional for metadata?

[Lestropie]

Relates to multiple existing issues; mostly https://github.com/bids-standard/bids-specification/pull/1003, #65, #58.

Imagine an MRI BIDS dataset, which contains a T1w image and a single fMRI free run. Neglecting the Inheritance Principle altogether, it might look something like:

sub-01/
    anat/
        sub-01_T1w.nii.gz
        sub-01_T1w.json
    func/
        sub-01_task-rest_bold.nii.gz
        sub-01_task-rest_bold.json

There are many metadata fields that will be identical between these two images; things like scanner site, field strength. It would be nice to be able to use the Inheritance Principle to define this information just once in a file sub-01.json; ie.:

sub-01/
    anat/
        sub-01_T1w.nii.gz
        sub-01_T1w.json
    func/
        sub-01_task-rest_bold.nii.gz
        sub-01_task-rest_bold.json
sub-01.json

This however would not be currently permitted, as suffices are mandatory. Conversely, having a data file without any suffix is hard to make sense of, as one would expect some mandatory indication of what kind of imaging contrast is present in data.

So the idea I'm toying with at the moment, is to make it mandatory for data files to have a suffix, but optional for metadata. This would allow singular definition of metadata fields that are common across image data of different contrasts.