Perhaps Grim Jr gets lost and ends up in Minnie's neighborhood. Minnie, being kind - hearted, takes him in. As they spend time together, Grim Jr learns about friendship and love from Minnie. And Minnie, in turn, discovers new things about the spooky world Grim Jr comes from through his stories. Their relationship could develop as they face challenges together, like a mean - spirited character trying to cause trouble in Minnie's world.
One possible plot could be that Grim Jr, being his mischievous self, stumbles into Minnie's world. Minnie is initially cautious but then they start to bond over their different outlooks on life. They could go on adventures together in Minnie's colorful world.