With some file formats supporting data across modalities (any volumetric data can be NIfTI, any raster image can be TIFF, anyting at all can be ZARR) I wonder if it makes sense to restrict these “extensions”.

I'm also wondering whether the terminology shouldn't be renamed to “formats”.

More generally, I'm also not sure why the emergence of a new format would need to be “accepted” by BIDS first before a dataset using it can be BIDS-compliant.
Is there any reason why we would ever say no?
If not, why not allow any data format?

I'm mentioning data format specifically, because for metadata files, which BIDS as a standard controls the contents of, we can't just have people using participants.xlsx. But BIDS does not control the analysis of TIFF, or NWB, or MNAF (my new amazing format), so why not let people use whatever fits their use case?

I see some utility in discouraging bad practices, such as proprietary or .m files for everything, or compressed .jpeg for optical imaging — so maybe allowing anything would go too far. But in any case I think open formats with no compression could be globally accepted.