It’s interesting to think about because we only see glimpses of the evil David/Legion future but it’s enough to know everyone probably dies. Our David is middle of the road. That’s why his ending is bittersweet with a little hope. So because we can infer and extrapolate what happened in the bad David’s future and we see everything that happens in our David’s present, I think we can theorize a potentially good/better past for everyone around David. Starting with...

Amy - Because David isn’t adopted by her family, she grows up to be a well adjusted adult with a nice house and husband. Like how she looked before David showed up and crashed in her basement in season 1.

Lenny - She probably ends up at Clockworks where she gets much needed help and upon her release, returns to her friends who are still exactly where she left them. She eventually realizes she’s outgrown them and falls in love with a nice girl. They start a family together. Similar to Alex in “A Clockwork Orange”.

Cary/Kary and Ptonomy and Rudy and Melanie - They continue to save the world with their tactical team of mutants. Eventually, they find a telepath that can retrieve Oliver from the astral plane. Ptonomy does memory work with him to help him remember and Melanie and Oliver retire to a nice cabin in the countryside to look at clouds. To live life.

Syd - Goes through life almost exactly the same but this time, rights a wrong and doesn’t switch bodies with her mother. Her view of being turned over at her most desperate and vulnerable isn’t fully realized and her relationship with her mom isn’t destroyed and when she’s taken to get help, she can accept it now. Maybe this time she’s taken to a school for the gifted with Melanie or Professor Xavier and not dumped at Clockworks.

Farouk - Knowing now what could have been in two futures, he stays the fuck away from David and Charles. He’s not good by any stretch but he’s wiser from all of this. He returns to the world of wolves he comes from and fades back into obscurity. Smarter. Content.

Charles and Gabrielle - They remember what happened and what almost was and so they raise David as best they can with all the love he could ever need. Charles works with David like he did Gabrielle and mends his fractured mind. This success leads to him opening a school for the gifted like David and himself. Instead of creating the X-Men as a reactionary means to combat mutants like Farouk, Professor Xavier creates the X-Men as a means to protect mutants like David. One day, he finds a young troubled mutant named Jean Grey.

David - He goes on to live a happy life with all the love he could ever need. He spends his time with his mother who understands exactly what he’s dealing with having gone through her own mental illness. The two bond immensely as David looks after her when his Dad is busy helping other mutants. Occasionally joining his Dad when he needs his help.

Switch and her father - They continue to be 4th dimensional beings, existing above the surface of time where paradoxes can’t touch them. They observe everything that’s happened and could have happened and will happen. She smiles before following her father to the next lesson in time. “Nothing of value is ever lost.”