Considering that all Trainformers come from Japan ('cept for Astrotrain), I wouldn't hold my breath on seeing a Tangara.I'd like to see a train-TF that either transforms into or comes with a realistic passenger carriage! The carriage could become a combat deck or combine with the engine to form the robot mode. Or it could be another Transformer altogether, but a carriage-former would be hard to market/sell to most folks (cos who would buy a TF carriage on its own?) unless it came packaged with another TF that transformed into the engine... but that would make it more expensive.
The G1 Micromaster trains had carriages which were basically their gestalt component/weapons - they weren't passenger carriages. I would like future train TFs to have couplings that are compatible with the CR/RiD trains' couplings so that it could attach onto say J-Five (Railspike) etc.
Now Archer argues that trains don't sell well in Hasbro markets but how well did the Railracer Autobots sell in North America? (shame they never came out here) As I recall, Universe Railracer (Hasbro repackaged reissue Sixtrain) sold reasonably well here. And the G1 Transformers electronic train set seemed really popular in the 80s (and they're not even really Transformer action figures).
I think train Transformers can sell well here... it's not so much an issue of alt mode but an issue of execution. If you make a train TF and you do it well, then people will buy it and like it. Look at Classics Astrotrain. Even as a triple-changer with a 'compromised' train mode it still sold reasonably well. Granted part of its appeal is the G1 Astrotrain character, but again - Universe Railracer - that seemed to sell well didn't it? And that mould was 11 years old when it came out (and let's face it, are pretty ordinary toys).
With a few exceptions like plants and humans (as much as I would love a Botanica toy, I don't think it would market/sell well no matter how well engineered it was) I think you can make a Transformer transform into anything and it would still sell well so long as the toy is well designed and well made!
Look at theFootpath PatrolStreet Action Mini-Con Team; you have a Vespa scooter, push-bike and a skateboard... yet they're pretty well designed/made toys considering their size and price-point (and the fact that they're a gestalt).
A skateboard! Once upon a time people would have laughed at the suggestion of a skateboard Transformer.