One idea could be a crossover with the Legend of Zelda series. Link could be a special guest in the Mario Party world, bringing his unique items like the Master Sword and Hylian Shield. He might participate in mini - games that require his combat skills or puzzle - solving abilities.
If it's a crossover with Final Fantasy, Cloud Strife could be a main character. He could bring his sword - fighting skills to the Mario Party world. His serious and brooding personality would contrast with the more light - hearted Mario characters, creating some interesting interactions during the story and mini - games.
You could do a 'Time Capsule' custom story. Have everyone bring an item that represents a memorable moment in their life. Then, during the party, each person shares the story behind the item. It's a very personal and meaningful way to get to know your guests better. For example, someone might bring a ticket stub from their first concert and talk about how amazing it was.
One idea could be to have a hero-themed party where the characters from My Hero Academia all show up in their best hero costumes. There could be a competition for the best costume, with prizes like limited - edition hero merch. Another aspect could be a battle - royale - style game where they use their quirks in a safe, party - friendly way, like shooting small blasts of confetti instead of real power blasts.
One idea could be Naruto getting transported to the Mario world. He might use his ninja skills to help Mario in his adventures, like using his speed to outrun the Bullet Bills. Another could be Mario somehow ending up in the Naruto universe and trying to navigate through the Hidden Villages using his jumping and power - up skills.
One idea could be that Naruto gets transported to the Mario world. He might use his ninja skills to help Mario and Luigi in their battles against Bowser. For example, Naruto could use his Shadow Clone Jutsu to confuse Bowser's minions while Mario goes for the power - up.
You could write a Paper Mario fanfic that involves time travel. Mario goes back in time to prevent a great disaster that happened in the past of the Paper Mario world. This could lead to him interacting with past versions of characters and changing the course of history in unexpected ways. Or, a fanfic about a new villain that only exists in the paper realm and has unique powers that Mario has to figure out how to defeat.