To compare previous training(s) (done by us or someone else, no matter).
Field with opponent's distance in the "parameters" line (or somewhere else, to distinguish from "ours").
Opponent's data taken from .gpx file (or any other internal format). Checking if same route would be nice, but not really necessary, distance check would be enough. There must be an option to save current training parameters (besides entries for analysis) to be compared in next sessions.
It would be nice to show opponent position on the map.
Next step.. Multiple opponents :)