Unfortunately an FM Transmitter is a rarely requested feature. It's also not legal to use such phones in many countries (some you would need a license for). I know in the UK as recent as 10 years ago these devices used to be illegal. Maybe that's changed now. So there's not even a worldwide market for it.
Also this technology has been replaced by the likes of BlueTooth (which nearly all new phones have), even DLNA. Many entertainment systems in new cars come with BlueTooth connectivity.
My point is, you may be waiting a loooonnng time for another phone, Android or otherwise, to come out with an FM Transmitter.
Still, I wish my phone had one!