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I hope that it will completely replace SMS and MMS messages for the contacts that are on iOS devices in the messages app. So that if the recipient is using an iOS device you won't need to do things differently, but are given the improvements of encryption, not the limiting word count on SMS (usually not a problem these days unless you're paying for them individually or sending them to certain people with older phones that can't fuse the SMS together), or the limited image quality of an mms.
It would also be nice if they opened this up for developers to integrate other messaging systems like whatsapp, viber or different IM-platforms, which would allow you to send messages to anyone regardless of what network they're on.