Please note that GPX is not a true replacement for FIT or TCX. GPX was born to capture GPS coordinates, not to model fitness activities. There is no direct / reversible mapping from a TCX to a GPX, and most of the data inside a FIT file won't fit :-) in a GPX file.
The approach I've adopted in my apps is basically the same approach adopted in Garmin Connect: export only GPS and time coordinates. This is what you really need to use mapping / geocaching apps.
If you want all the data, stick with the native format :-)