Perhaps Wally returns with a mission from another dimension. He has to convince the Young Justice team to help him, but they're hesitant because they don't fully understand the situation. This could lead to a great adventure where they learn about new threats and also about Wally's experiences in the other dimension.
One idea could be that when Wally returns, he has new superpowers. Maybe he gained the ability to manipulate time during his time away, which he has to learn to control. This could add a whole new dynamic to the team as they deal with the implications of such a powerful ability.
Another plot could involve Wally's return causing a rift in the team. Some members might be overjoyed, while others are suspicious of where he's been and what he's been doing. This internal conflict could be explored as they try to decide whether to trust him again.
Well, it could be that Wally returns with new superpowers. Maybe he was in some sort of time - warp or alternate dimension and comes back stronger. He might have to prove himself to the team again that he's still the same Wally they knew.
Wally could be developed by showing his growth in character. For example, if he was a bit cocky before, he could return more humble and with a deeper understanding of the world. This change could be a result of his experiences while he was away.
A plot could be that Wally is cursed by an ancient magic and Doctor Fate is the only one who can break it. So, Wally has to follow Doctor Fate's instructions and training to get rid of the curse. Along the way, he discovers new aspects of his powers and also about the magical side of the Young Justice universe.
Maybe Wally returns with new superpowers. He could surprise his old friends in Young Justice and they all go on a new mission together.
In a 'Young Justice Outsiders Wally Returns' fanfiction, he might return as a changed person. He could have lost his memories while away and slowly regain them as he interacts with his old friends. His return could also bring back old threats that were thought to be gone. For example, an enemy from his past might resurface, believing that Wally holds the key to something they want.
One idea could be a time - travel plot where Fem Robin and Wally end up in a different era in the Young Justice universe. They have to work together to find a way back while also dealing with the challenges of that time period.
One idea could be that Kid Flash returns with new superpowers. Maybe he was in some sort of other - dimensional training and now he can phase through time as well as objects. This would add a whole new dynamic to the team in 'Young Justice' as they deal with threats that require time - traveling abilities.
Well, 'Speed Force Adventures of Wally West' is an exciting fanfiction. It shows Wally using his speed in really creative ways during Young Justice missions. 'Wally and the Team' is also a good one. It's about how Wally interacts with different members of the Young Justice team, like his banter with Robin and his mentorship - like relationship with some of the newer members. And 'Wally's Time - Out' is interesting as it explores a period where Wally has to take a break from heroics and how he deals with it.
Maybe a villain from the dimension where Wally has been. This villain could have followed Wally back to seek revenge or to take over the world that Wally has come back to. Since Wally has some knowledge about this villain, it would be his responsibility to stop the villain and protect his friends and the world. The villain might have powers that are a match for Wally's, making it a really tough fight.
One interesting 'Young Justice Wally fanfiction' could be about Wally's adventures in an alternate dimension. In this story, he might have to face different versions of his friends and foes, and learn to adapt his speedster abilities in a whole new way. It could explore his character growth as he tries to find his way back home.