It could be alternate versions of the villains from their own universe. For instance, an alternate Voldemort who is even more powerful. Maybe there are also new characters who oppose Harry and Teddy just because they are outsiders in this alternate universe.
In this fanfic, they might encounter alternate versions of themselves. For example, Harry could meet a version of himself who is a dark wizard instead of a hero. Teddy, with his shape - shifting ability, could have a hard time adjusting to the new universe where the rules of magic might be different. They may also find new magical creatures that don't exist in their original universe and have to figure out how to interact with them.
Perhaps he meets a version of his parents who are still alive. This would be an incredibly emotional encounter for Harry. He would get to know them, and they could teach him things about magic and life that he never had the chance to learn before.
Well, often in these fanfics, a major plot twist is the technology or magic system. In the RWBY universe, Dust is important. But in the alternate universe, it could be that Dust has no power, or it has some completely different and unexpected effect. And the semblances of the characters might change drastically. Say, Yang's ability to absorb damage and use it as strength could turn into something like the ability to heal others instead. And then there's the world - building aspect. The alternate universe could be a post - apocalyptic wasteland instead of the somewhat civilized world we see in RWBY, which would be a huge plot twist for the characters to deal with.
One idea could be a castle that floats in the sky in the alternate universe. It could be made of some kind of magical crystal that gives it the ability to levitate. The inhabitants of this castle might be a race of winged beings who use the castle as a base to protect their realm from dark forces.
Well, Hermione is often depicted in different ways in these fanfics. In some alternate universes, she could be a powerful dark witch instead of the studious and good - hearted character we know. Ron might be a super - confident and arrogant character rather than the sometimes insecure boy in the original story. Also, Snape could be a completely different person. He could be a jolly and friendly teacher rather than the brooding and complex one we are used to.
It's possible that they would accidentally disrupt the timeline in the 'Harry and Teddy go back in time fanfic'. This could lead to all kinds of unforeseen consequences. They might create new threats or change the relationships between different characters in unexpected ways.
He might meet different versions of his friends. For example, a Hermione who is a powerful sorceress ruling a magical kingdom. It would be exciting as he could learn new magic from her and be part of her adventures in this alternate reality.
One interesting concept could be a world where Voldemort won the war. In this alternate universe, Hogwarts might be a very different place, with dark magic being taught openly. Another concept could be a universe where the houses are not divided as they are in the original. Maybe students are grouped based on different criteria like magical talents other than just bravery, intelligence, etc. For example, there could be a group for those who are good at magical creatures handling.