I thoroughly enjoy Garmin Connect; nothing is better for tracking my outside rides and I LOVE my Garmin hardware. I also love tracking metrics for my rides, planning rides and training indoors in the winter. With that use case in mind I have a couple thoughts about what I would like to see in upcoming versions of Connect:
1) Workouts & the Calendar: Creating workouts is fantastic! Adding those to a calendar is even better. What would make this a killer feature is when the workout is performed with or without a device, it is synced on the calendar. for half the year I ride inside. I add my workouts to my calendar to plan and upload the .TCX from my trainer software. In the Calendar I have the planned ride and the actual Activity. Merge the two for ONE entry in my calendar
2) Support exporting/syncing calendar via ICS and RSS for the calendar so we can see planned workouts in personal calendars.
3) Body Composition: support manual entry of body composition
4) API: better support to add data via third party sites using your API.
That's all I can think of right now, otherwise, please keep up the good work!