Imagine. Its the nintendo direct. During it, they have a segment talking about a bunch of tougoi games coming, like the fighter, skydrift, and the new offical game, plus some other fan games. Then, they say they have a special exclusive touhou announcement.
It opens fully animated, done by either studio trigger, or one of groups that did those fan touhou animations. Flys over towards hakurai shrine, where reimu is drinking tea on the porch. She then hears someone by the donation box, and goes to investigate. Before she can see what was put in, Marisa flys over, saying a incident has started. Reimu looks over and sees bright explosions happening in the distance. Suddenly a bright laser flies towards her and marisa, who dodge it easily, but it blasts the donation box. Reimu pulls out her orbs and flies up towards the source. As she does the camera stays back, and what was in the donation box floats into the view... and its an envelope with the smash logo on it.
A smash remix of reimus theme starts to play as She comes to the source and is Galeem, and its firering lasers at other touhou characters, but they are dodging easily. Reimu shoots some danmaku at the entity who takes the hit. It flys back, and suddenly Mario, Zelda, and pikachu appear. Reimu smirks, and flys at them, ready to fight.
It then goes into gameplay, with reimu and Marisa talking about whats going on, who these strange people are, and just fighting. Remiu and Marisa speak japanese with subs btw. Marisa also appears as an assit trophy, and the stage Hakurei shrine is shown, which has cameos from Yukari, Suika, Tenshi, and others who jump in and attack everyone with various attacks.
It ends with back with animition, with reimu facing off against Galeem, and suddenly the other one shows up to. Reimu sighs, looking annoyed, and grabs a smash ball, activating her final smash and blasting the 2 beings.