The ability to extract part of an activity, rather than a full split, would be useful. My use case is I hiked around a lake, and had a swim half way through. I wanted to save the entire Hike as one activity, with the Swim as another. This is currently achievable, but it's long - split the activity, crop the middle activity, download all 3 files, use a text editor to piece gpx files 1 & 3 together, delete all 3 activities, upload the 2 new gpx files.

This could be made much easier by enhancing your current Activity Split tool.