Instead of providing only the
final rerecords counter of a TAS, I think it'd be interesting to display it for each frame (or a larger time frame, e.g. using the data of the last 2 seconds).
This would allow the viewer to identify which sections were more difficult to create.
As for the file size cost: for a binary-based TAS file structure, it'd be easy to just use
4 bytes per frame so the size increase would be ~14 KB per minute for a 60fps game.
Assuming a text-based format, it might be better to store only the rerecord counter
changes. This would reduce the data to a single "0" for most of the frames, i.e. there would essentially be a 1 byte per frame cost for file formats where the counter can be simply appended to the existing data (e.g. BizHawk's *.BKM) or 2 bytes per frame for file formats that require a separator (e.g. | or Tab). File size cost would be ~3.5 KB or ~7 KB per minute.