One possible plot could be that Mario and Luigi are exploring a new world and accidentally stumble upon the Pokemon world. They team up with Ash and Pikachu to stop Bowser who has teamed up with Team Rocket to steal all the Pokeballs and use the Pokemon's power to take over both the Mushroom Kingdom and the Pokemon world.
A really cool plot could involve a Pokemon tournament being held in the Mushroom Kingdom. Mario and his friends are excited to watch. However, some dark - type Pokemon start causing chaos during the tournament. Mario realizes that there is a hidden force at work, something that wants to disrupt the harmony between the two worlds. So, he teams up with his Pokemon friends like Charmander and Squirtle (who he befriends during this chaos) to find the source of the problem and restore peace.
Well, perhaps Bowser somehow makes a deal with some of the more malevolent characters in the Undertale world, like Flowey. Then Mario has to not only deal with Bowser but also the new threats from the Undertale realm. He might have to learn about the concept of LOVE and EXP in the Undertale world to fully understand how to defeat his enemies. And along the way, he could meet Sans and Papyrus, and they could have some really funny exchanges due to their very different personalities.
Maybe the Mushroom Kingdom is in trouble. A strange magical force is draining all the power - ups. Mario discovers that this force is linked to Equestria. He travels there and meets Rarity. Rarity is impressed by Mario's unique style and decides to help him. They find out that a corrupted magic crystal in Equestria is causing the problem. So they have to find a way to purify the crystal. Along the way, they face many challenges, like dealing with jealous unicorns who don't want an outsider like Mario interfering in their world.
A great plot could involve a new threat that is a combination of the dark forces from both universes. Naruto and Mario find themselves in a strange new dimension where the rules of both their worlds blend. They must learn to trust each other and combine their unique skills. For example, Mario's invincibility star could enhance Naruto's speed and strength during battles, and Naruto's sage mode could help him sense hidden traps in the Mushroom Kingdom like he does in his own world.
One possible plot could be that Mario and Kirby's worlds are somehow merged due to a powerful villain's spell. They have to team up to find the source of the spell and break it. Along the way, Mario could use his jumping and power - up skills while Kirby uses his ability to copy enemies' powers. They might encounter challenges like a new type of Goomba that can fly like some of Kirby's foes.
One interesting plot could be that Bowser teams up with the Octarians in the Splatoon world. He wants to use their ink technology to take over both the Mushroom Kingdom and the Splatoon world. Mario has to then form an alliance with the inklings. He travels through different ink - filled levels, learning new skills like swimming in ink and using ink - based weapons. Along the way, he discovers secrets about the connection between the two worlds. They finally face Bowser and the Octarian leader in a huge battle where Mario combines his classic moves with the ink - powered attacks of the inklings to save both worlds.
Imagine that Michael Rosen is from a parallel dimension where everything is made of words. When he enters Mario's world, he starts to turn the enemies into harmless words. Bowser, seeing this as a threat, tries to stop him. Mario and Michael team up. Michael uses his word - changing ability to weaken Bowser, and Mario gives the final blow. In the end, they learn from each other and Michael returns to his word - dimension, but not before leaving behind some magical word - seeds that grow into beautiful story - trees in the Mushroom Kingdom.
One interesting plot could be that Ben, using his Omnitrix, accidentally lands in the Pokemon world. He teams up with Ash and Pikachu to stop an evil organization that is trying to use both Pokemon and alien technology. For example, they might be creating hybrid creatures using Pokemon DNA and alien powers. Ben's different alien forms could have unique interactions with the Pokemon. His Four Arms form could be really strong in Pokemon battles, while his XLR8 form could be used to quickly traverse the Pokemon world to reach different areas in time to stop the villains.
One possible plot could be a trainer in the Pokemon world discovers a strange worm - like Pokemon that has abilities never seen before. This Pokemon could be a key to unlocking a hidden power within the region that an evil team is also after. The trainer has to protect the worm - like Pokemon and use its unique powers to defeat the bad guys.
Maybe there's a villain in the Pokemon world who is using some kind of Pokemon - related power to cause chaos, similar to the villains in BNHA. The heroes from BNHA have to team up with local Pokemon trainers to stop this threat. They discover new Pokemon abilities that they can incorporate into their own fighting styles. For example, Todoroki could learn from ice - type Pokemon how to better control his ice powers.
One possible plot could be that Sonic and his friends accidentally end up in the Pokemon world. They team up with some Pokemon trainers to stop an evil villain who is trying to use chaos emeralds and powerful Pokemon to take over both the Sonic universe and the Pokemon world. Sonic's speed could be used in Pokemon battles, like quickly dodging attacks or getting to strategic positions on the battlefield.