One idea could be that the characters from Undertale find themselves in the Deltarune world and have to adapt to its different rules and characters. For example, Sans could meet Ralsei and they could have a humorous exchange about the nature of their respective universes.
Perhaps a plot where the dark fountains in Deltarune start to affect the world of Undertale. The heroes from both worlds would then have to team up to stop the spread. Kris could teach Frisk about the unique combat mechanics in Deltarune, and they could use that knowledge to fight off any corrupted beings. This would involve a lot of character development as they learn from each other's strengths and weaknesses.
Another plot could revolve around a mysterious entity that has the power to merge the two worlds. Some characters are excited about the possibilities, while others are afraid of losing their own identities. Asriel from Undertale and Lancer from Deltarune could form an unlikely friendship during this chaos and work together to either stop or encourage the merge, depending on what they think is best for their friends and families.
Look to Chapter 230 for the clearest answer. It begins with Shang Shaojing's expression wasn't looking too good either., keeps building through He had always been the winner in any field, and there had never been an..., and lands on If he knew that your stepmother was secretly prompting you.
The best chapter pointer here is Chapter 238, "The Guests Were Dumbfounded, The Audience Was Excited". It opens with Jiang Li was actually first on all three leaderboards?! and closes on And, to Zhou Xu's alarm, they did find a problem..
The best chapter pointer here is Chapter 241, "Thieves Will Shed Two Tears When They Break In". It opens with It wasn't the director," Jiang Yan said with a blank expression, "Did you... and closes on However, as he turned around, before he could see the person properly, a....
The best chapter pointer here is Chapter 247, "Madam, Your 2000 Kilograms of Sweet Potatoes, Please Check and Accept". It opens with Facts have proved that arch-enemies often understand each other the best. and closes on Why should he expect food without contributing?] [Support!.
The best chapter pointer here is Chapter 235, "Bungee Jumping in Ethiopia???". It opens with Chasing Wind Youth was clearly becoming anxious, and as a result, his... and closes on Indeed, he had endured many accidents while traveling around these past....
If you want the clearest chapter reference, go to Chapter 235. Early on, Chasing Wind Youth was clearly becoming anxious, and as a result, his...; later, the chapter leans on I, I didn't, I'm not, I, I haven't been involved in illegal activities in....
A practical answer is Chapter 213, because it turns the idea into a concrete scene. The chapter opens with Jiang Cheng also noticed where Jiang Shixu's attention lay., adds pressure through Director Sun, please don't tell me the King for today is me again., and ends on As soon as they were seated.
The best chapter pointer here is Chapter 243, "How Could It Be Her?!". It opens with Hahaha, the young master's face has gone white, but I got a scare just now... and closes on Perhaps feeling that the atmosphere had become unbearably tense, Director....
The best chapter pointer here is Chapter 257, "Did Jiang Li Get in Through the Back Door?". It opens with This nation seemed to have a particular love for bright colors. and closes on Yet in the end, not only did she have no sense of guilt, she even forgot....