For the record, Raiden makes the most sense as a first combiner from an engineering perspective. The individual trainbots are already large, and parts form in animation, thus solving a huge (ba-dum-tsss) problem with combiners. They also have a non-traditional combination scheme ala Devastator, making it super easy to make all of them the same height. He’s also big and chunky with relatively simple individual transformations, eliminating possible stability problems. From a marketing perspective, a huge set of somewhat accurate trains at what will probably be pretty close to model scale nabs takara a massive market that wouldn’t normally look at their product. The last Japan only character they did (starsaber) was also one of the most publicized releases they ever had, and topped the toy sales on Amazon.jp for about a week.