Perhaps Si is transported back in time to when Summer was alive. Si then has the chance to interact with Summer directly. They could become friends and go on adventures together. Along the way, Si could learn valuable lessons from Summer and also change some small events that have a big impact on the future of the RWBY world.
A great plot could be centered around the Summer Maiden's relationships. For example, she might have a forbidden love with a character from the other side, like a member of Salem's faction. This would create a lot of internal conflict as she has to balance her feelings with her duty as a Maiden. Or perhaps she has a deep friendship with a character who is in danger, and she has to use her powers to save them from a great threat.
One idea could be a story where Pyrrha and Summer are brought back to life in an alternate reality. They then have to team up to face a new, powerful threat that is endangering their world. Another plot could involve a pre - RWBY story where Pyrrha and Summer meet during a training mission and learn from each other's skills.
One idea could be that Summer's brother has a unique semblance that he has to keep hidden from the enemy. For example, it could be a semblance related to time manipulation, which is extremely powerful but also very dangerous if misused. He might be on a journey to discover more about their family's past and the secrets it holds, which could be linked to the greater mysteries in the RWBY world.
Another plot could be that they accidentally travel to a future time instead of the past. In this future, Remnant is in a state of chaos and they have to figure out what went wrong. They might discover that it was their own actions in the past (from their original timeline) that led to this future. So they have to find a way to go back to their original time and change things to prevent this dark future from happening.
She could return as a sort of spirit guide, not fully corporeal. This would add an interesting dynamic as she can't interact in the physical world in the same way. Her return might also be linked to a greater threat. Like she has seen the oncoming danger in the afterlife and has come back to help the team prepare for it. It could also lead to revelations about Ruby's true potential as she inherits some of her mother's latent powers.
A plot idea could be that Ruby time - travels to become Summer right before a major event in Summer's life. She then experiences all the things Summer did but with her own mindset. She might try to change how Summer interacted with certain characters, like Ozpin. Maybe she would be more open with him about her plans or suspicions. This could lead to a different chain of events. Also, Ruby could use her knowledge of Dust and its uses in the future to enhance Summer's fighting abilities. And in the process of being Summer, she could also discover some long - lost secrets about her family that were never revealed in her own time as Ruby.
The summer dragon could be the key to saving a kingdom that is suffering from an eternal summer. A group of adventurers sets out to find the dragon and convince it to use its power to restore the normal cycle of seasons. Along their journey, they face many challenges like treacherous terrains and other dangerous magical beasts.
One idea could be a time - traveling adventure where Summer and Morty end up in a parallel dimension where their roles are reversed. Morty is the confident, popular one and Summer is the nervous sidekick for a change.
One idea could be a new student arriving at the school with a mysterious power that no one has seen before. This could cause both excitement and suspicion among the other students.
One idea could be a mercenary who is hired by a mysterious client to steal an ancient relic from Beacon Academy. He has to navigate through the various defenses and teams of Huntsmen and Huntresses.