I know it's one, but I can't decide! One of the three below.
3rd party video support, uploading, conversion, streaming
Google calendar api integration, fetch google events to populate the empty calendar
Subscriptions earned for activity, see: https://invisioncommunity.com/forums/topic/464067-subscriptions-earned-for-activity/
Then there are these other ones...its just too much to wish for I guess!
Targetting page elements with in different forums views
RSS import improvements, more content and media imports. RSS Enclosure Tag, Other File Types like audio/mpeg. Things like RSS Import Image as Blog Headers, add tags, etc
New notification when content is approved (from 4.6)
Subscription enhancements like coupons
Built in newsletter feature, scheduling, creation, subscription