Back in June, there where rumors going around that iMessage for Android was going to be announced at Apple's Word Wide Developers Conference. Unfortunately, that announcement never happened. However, it seems like Apple has been working on "mockups" of its iMessage application for Android, which possibly means that the company at some point seriously considered bringing its popular messaging application away from being exclusive to iOS.
According to Apple-focused writer John Gruber, its been rumored that their are mockups of iMessage for Android that have moved around within the company. The mockups have included different user interface styles including looking like the iMessage app found on iOS or a more Material Design focused app. For many current iPhone users, iMessage is their most used application on there device. Apple handles more then 40 billion messages per day and expanding it to Android will allow them to reach more users and collect more data.
Google already has its fair share of messaging applications though. In fact, its latest messaging platform, Allo, which features the Google Assistant, has been very popular having been downloaded over 5 million times. According to John, iMessage for Android may never be released but its important to note that the existence of the mockup suggests that their still is a possibility. However, Apple makes the majority of its money when people buy its hardware and people purposefully buy Apple products just so they can use iMessage. Its hard to see Apple give up that hardware advantage. A "Senior Apple Executive" told the Verge in early 2016 that Apple's user base of over 1 billion devices is enough for any AI learning the company is currently working on and that a messaging application that was exclusive to Apple is key to helping with its hardware sales. Also, because Apple's hardware helps with its encryption, there might be security problems porting it over to Android.
It's nice to know that their still is a possibility that fans of the iMessage app might be able to use it on their Android devices. With applications like Apple Music on Android sporting material design, it's not unreasonable to think that more Apple made Android apps are a possibility.