Your idea

In fMRIPrep, we generate _boldref.nii.gz files from some combination of the source bold file, an associated sbref (if available) and within-space transforms (such as SDC warp). This is used for performing SDC, coregistration, visualization, and as a resampling target for a full BOLD series.

I was initially going to put this into #519 as _boldref and _cbvref, but is it more general? Does this make sense to put into Imaging data types?

Would something like the following ever be generated?

• _dwiref (cc @oesteban, @arokem, @Lestropie, @bids-standard/derivatives-mri-dwi)
• _aslref (cc @patsycle, @HenkMutsaerts)
• _petref (cc @mnoergaard @melanieganz, @bids-standard/derivatives-pet)