Rumors are that Apple is also working on money services such as sending money from one iMessage user to another. Kinda what WeChat has been doing for ages only that nobody outside of China really cares. If that were true then multiplying ones potential user base by adding Android to the game would be a rather smart move.
If this turns out to be true it'd be amazing. iMessage and FaceTime are the reasons why I love my iPhone. The only problem I see is that people would have to actively download it from the Google Play Store, which truth be told I don't see happening. If, in the other hand, it would come preinstalled on some devices as part of either the OEMs or the carriers bloatware package it might actually gain a significant user base among Android users as well.