According to several user tweets, pressing the plus button opened up their camera roll to select photos, but failed to let them upload to their feeds.
This feature was then swiftly taken away in another update, but if we had to guess, either Instagram is preparing to roll out a multi-photo feature that can display like a Carousel Ad, or the button was mistakenly pushed to regular users, not approved advertisers.
— Umpf (@Umpf) March 1, 2016
Other user speculations for the button include an album feature, a video/photo reply button, or the option to repost the photo on your own feed (which users have been doing via third party apps.) We’ve reached out to Instagram for comment, and will update if we hear back.
What do you think it could be? Better yet – what do you want it to be?