Create a function to a selected track (by a clip or otherwise) becomes solo. Cubase has this feature (optional). To listen (and editing), it's fantastic. This is faster than: click + "shift S".
It would be amazing to see some helpful composer tools added to Protools.. Much like Cubase.. Expression mapping would be great. Also Parabolic Midi curves rather than having to do it with a straight line etc.. Also - Some view options to see only tracks with regions on them i a selected area. I am sure there are plenty more. Just look at cubase and copy them.. That would be awesome :)