I would like some additional categories in the Activity Types in Garmin Connect. It seems weird to have things like wind/kitesuit flying but not have more common activities like Yoga or Pilates. Even something like Martial Arts would be helpful to many people.
My suggestions:
Yoga - Break this up into Hot Yoga, Vinyasa, and other types of Yoga as well.
Pilates
Martial Arts - Karate, Jujitsu, as well as other types like Fencing or HEMA
Boxing - Split into categories for Heavy Bag, Drills, Sparring, etc.
Sauna
Being able to edit your own categories would be helpful to.
Adding to the 'Compare Activities' would be helpful for tracking progress. For instance showing the heart rate graphs matched up either by distance or by time so you could compare your runs or when doing a similar workout over again (like in Crossfit).