Apple is opening up their messaging platform to developers, so you can do things like pay a friend with SquareCash, send a custom sticker, or collaboratively order food.
Media like music and links to websites are finally going to look better in iMessage. The app will pull in previews of that you can play or view from within the app.
Bigger and Smarter Emoji
Emoji will be three times larger. Everyone loves emoji. They’ll also get easier to use, as iMessages will now look at what you’re typing and suggest some relevant emoji.
Spice up a few of your messages with an explosion of sound and color, that take up the entire iMessage window. (This will probably annoy everyone.)
Writing on images
You’ll be able to take pictures without leaving iMessage (the keyboard turns into the camera window), and then write a message on the picture you take.
Invisible Ink, Hidden Photos
Write a message or send a photo that is initially obscured, and the person receiving the message will use their finger to reveal it. This will certainly be used inappropriately.
Show all your friends your terrible handwriting, by writing them a personal note instead of typing.