My theory is that cap was getting cut. Mario didn't even send him an invitation. He was pissed and went to the smash stadium to complain. Meanwhile chrom had just arrived along with robin and Lucina for moral support. Cap didn't like that another swordsman was chosen over him. So he went to beat him up to prove he was better. Mario came to help along with link samus (zero suit) Kirby and pikachu. At 1st samus pointed her blaster at him shouting "that's ENOUGH Douglass!!" But Mario liked wat cap was doing so allowed it to continue. He declared that the winner will be in smash. Of course cap won but he didn't stop. Lucina intervened to defend chrom and that's where the trailer starts. Mario liked the way Lucina defended chrom and he liked robins fighting style so he decided to allow both of them to be in smash (but as per the agreement chrom was cut and cap too his place) but robin pulled some strings to let chrom be apart of his final smash
I wanted Chrom so badly. He was my most wanted character. Oh well, I'm happy that we got 2 Fire Emblem characters instead of 1. I like Lucina and Robin. Also, I'm happy Chrom is in Robin's final smash. I might even main Robin. What's your opinion on the characters.
I guess I'm a little bummed out that you can't use Chrom in Smash. But I guess I'm really happy that they included one of the most unexpected character in the game. Most likely I want to be able to use Robin, just so I can see his/her final smash with Chrom.