It would be a good option Matt, but if you are looking into it, could it be more generic and look for Grouping changes?
I guess that would only apply to the "=Counter()" method, and would need an additional parameter in the function. The application might be if someone wanted to create a Custom field similar to Sequence number, for sorting of a group of tracks, external to Playlist sorting/sequencing. Or maybe sequentially numbering a list of images, podcasts, etc. The new parameter may specify the actual Group to switch on, but I would be happy if it just used the Grouping and Sorting within Group that a View already displayed.
The "Fill track # from list order" only ever applies to Track numbers within an Album and Episodes within a Season, and it would be great to add the option in that feature, as it is easy to understand and straightforward to use. But the "=Counter()" method is more universal.