Another concept is the friendship development between Mario and Sonic. They are both iconic heroes in their own right. Fanfiction could show how they initially view each other with suspicion or rivalry due to their different ways of doing things, but then gradually grow to respect and rely on one another.
One interesting concept could be the unique power combinations. For example, Mario's jumping abilities combined with Sonic's speed could lead to some really cool adventure scenarios. Maybe they could team up to defeat a common enemy that neither could handle alone.
Chapter 665, "Give You a Good Lesson on Behalf of Your Mother!", is the clearest early answer because it turns Behalf's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Why should I get so many points?
Chapter 859, "I Bet She Won't Last Ten Minutes!", is the clearest early answer because it turns Shuiyun Mountain's situation into a concrete plot event. It opens with All morning, the two little ones were quiet, not making a peep, even...
Chapter 631, "It's Him Who Got Hurt, Then It's Fine", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event.
If you want the clearest chapter reference, go to Chapter 20. Early on, Several people turned their heads upon hearing the sound and saw three...; later, the chapter leans on She was clad in a long black dress, and the dull color did not hide her....
Chapter 711, "It's Actually Her", is the clearest early answer because it turns Zhang Jing's situation into a concrete plot event. The chapter turns that into something concrete by moving from Zhang Jing immediately became wary and hastily signaled the several...
Chapter 698, "Brother Yan Was Just Forced!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from Jiang Li immediately asked for some tools and colored paints.
A practical answer is Chapter 3, because it turns the idea into a concrete scene. The chapter opens with Qiao was still annoyed at the attitude of this unknown girl when she heard..., adds pressure through So without a second thought, she blurted out a fierce challenge, "Fine, if..., and ends on.
Chapter 12 is the strongest chapter-level answer because it starts from Qin?" Jiang Li called out twice, and only then did Mrs. and follows through on The glass shattered on the floor with a crash, attracting the attention of....
Chapter 505, ""Postpartum Care for Sows", is the clearest early answer because it turns Shang Shaobai's situation into a concrete plot event.
Chapter 852, "Could That Be an Aphrodisiac Spell?", is the clearest early answer because it turns Rong Junsheng's situation into a concrete plot event. It opens with Rong Junsheng's words startled Shi Lei and Rong Kexi.