There was a time when Travis Touchdown was considered a good shout for a spot on the Super Smash Bros. Ultimate roster, and while he technically did make his way into the game as a Mii Fighter costume, the real collab actually ended up appearing in No More Heroes 3.
As those who have played the game already will know, No More Heroes 3 features a number of special matches which change up the game's usual formula, often taking on characteristics of other game genres or specific well-known titles. In danger of spoiling the game for anyone who hasn't finished it yet (head back to safety now if you'd like), the final boss battle is very reminiscent of Super Smash Bros., and it turns out that Nintendo was completely on board.
In an interview with Siliconera, creator Suda51 explains that both Nintendo and Smash series director Masahiro Sakurai actually gave him their blessing to develop the final scene as he liked:
"As for the Smash Bros. thing, that’s kind of a different story. Actually, I wrote that up kind of screwing around. It mainly started as a joke. I was gonna say 'OK, we’re gonna have FU and Travis take each other on in a sumo style fight, and hey! Let’s make it Smash Bros. style.' I wrote it up a total rip-off of the Smash Bros. style and we started developing it. When I saw it coming to shape, I thought this might be a problem because it was really Smash Bros.-y. It was really, really close to the actual game. I decided to change it, or completely get rid of it altogether.
But before I did that, I actually went and spoke to the people at Nintendo to see what they thought. I was kind of surprised that they told me 'No, do it! Go for it! We think it’s really cool and that it’s better like this. If anybody gives you s*** for it, we’ll take responsibility for it. We’re not gonna get mad. It’s fine.' Sakurai himself even said 'Yeah, this is cool. You have my blessing. Go for it.'
I was like… Really? Are you sure this is OK? And they were like 'No, definitely. Totally. Do it. Go for it.' So instead of changing it to a different style to not rip off Smash Bros. Nintendo basically told me to rip them off as much as I could. And that’s how that boss fight kind of came to be. It started out as kind of a stupid joke, and the people who mattered happened to like it not only supported it, but requested to keep it in."
You can see the fight itself in the video below from the 20-minute mark, but be warned that the clip features lots of bad language:
You can read Siliconera's full interview with Suda51 here.