Group messages is also being enhanced for iOS 8 thanks to new features. You'll be able to add and drop people from conversations and silence non-stop incoming message annoyances via a group-specific Do Not Disturb toggle.
Sharing your location for a set period of time is also going to be a part of iMessages, essentially forking over the concept from Apple's underused Friend My Friends app.
Location sharing, when it was part of the standalone app, was ideal for meeting up in a crowded location like a baseball stadium or concert, and now it'll get more use within iMessages.
Multimedia within iOS 8's iMessages app should be more useful too. Inline voice and video messages with Snapchat-like clips that self-destruct are coming to this mobile OS update.