I wrote a PoC (Proof of Concept) a few years ago of recompiling Xamarin iOS apps to Android without code changes and we released a few apps using that. It is especially handy for LoB (Line of Business) / internal apps which actually are very usable like this. Clients loved it as they only had to pay once. It has parts of the Core Graphics and Core Animation implemented as the last app I ported with it was heavy on those.

This was before Forms came along. No clue if anyone would be interested in it, but i'm cleaning up the code to put it on github as open source.