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.
It could be a new villain who has a vendetta against speedsters. Since Wally is a speedster, this villain might see his return as an opportunity to prove himself. Maybe he's a scientist who has developed a weapon specifically designed to counter speedsters' abilities.
Maybe Wally returns with new superpowers. He could surprise his old friends in Young Justice and they all go on a new mission together.
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.
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.
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.
The Reach could be a great antagonist. They might see Kid Flash's return as a threat to their plans for Earth. They could try to capture him and study his powers to find a way to counter the other heroes. Since they are an alien race with advanced technology and devious plans, they would pose a significant challenge for the 'Young Justice' team.
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.
There's this fanfiction called 'The Unspoken Bond'. It's about how Artemis and Wally communicate without words in the middle of battles. It shows their deep connection and trust. The story is full of little details that make you feel like you're right there with them, watching their relationship grow and strengthen.
In some fanfictions, Wally is often portrayed as a bit of an outcast due to his immortality. He feels different from his friends and teammates. His personality might change too. He could become more reserved as he witnesses the passage of time around him. But at the same time, he still has that underlying heroism that makes him a great character.
There are many great fanfictions out there about Wally and Artemis' love in Young Justice. One type often shows their relationship growing slowly. For example, they start as teammates who bicker a lot, but gradually realize their feelings for each other during missions. The fanfiction might focus on how they deal with their different personalities - Wally's more carefree nature and Artemis' guarded side. Another aspect could be how they keep their relationship a secret from the rest of the team at first.