I have seen this hinted at in other threads, but not as a specific request. As far as I can see you cannot currently do this in BaseCamp (or Mapsource).
It would be great to be able to change the style (thickness) of a tracklog, not only the colour. A choice of different line styles would be a "nice to have" as well.
Also the ability to display / not display trackpoints.
If you can do it in Google Earth, it should be possible to implement this in both BC and MS.