Well, for Anakin, his first reaction in the future could be one of confusion and a bit of awe. He'd see things he couldn't even have imagined in his wildest dreams. His mechanical parts might start acting up due to the different technology around. Obi - Wan would be like a sponge, soaking up all the new information. He'd be comparing this new world to the Jedi teachings and the past he knew. Padme would be fascinated by the new architecture and the way people interact. She'd start asking questions right away, trying to piece together how this future society came to be. And all three of them would be a bit disoriented at first, but their different personalities would drive them to explore and understand the future in their own ways.
In a fanfic where they go to the future, they might be shocked by the advanced technology. Maybe they see flying cars and buildings that touch the sky. Anakin could be intrigued by new forms of lightsaber combat, if they still exist. Padme might be amazed by the new political systems and how society has evolved in terms of equality.
Anakin may have a somewhat conflicted reaction. Given his past and the events that unfolded in his life, he might be a bit hesitant to embrace a fanfic about his kids. However, his love for Padme and his children would also make him curious. Padme would approach it with enthusiasm. She would be interested in how the fanfic captured the essence of their family. She might even offer suggestions or corrections if she felt certain aspects were not in line with what she envisioned for their family, all while being very supportive of the creative effort behind the fanfic.
Anakin might use his combat skills to survive. Padme could use her diplomatic skills to communicate with the future people. They would first have to learn about the new technology and customs.
Obi - Wan might be shocked at first. His eyes would widen and he'd quickly try to assess Anakin's condition, checking for any visible wounds.
Padme would most likely have an immediate emotional response. Tears might well up in her eyes as she sees Anakin burned. She would be in a state of disbelief at first, not wanting to accept that this has happened to him. But then, her natural instinct to care for him would kick in. She would use her influence to get the best medical care available, and she would stay by his side throughout his recovery, constantly reassuring him that he will be okay, even if she's not entirely sure herself.
One possible plot could be that Anakin and Obi - Wan find a time - traveling device in the Jedi Temple. When they travel to the future, they are shocked to see the remnants of the Jedi Order. They meet new Force - sensitive individuals who have formed a resistance against a new dark side threat. Anakin, having redeemed himself in the past, uses his experience to train these new Force - users, while Obi - Wan tries to figure out how the Jedi Order fell so far and how to prevent it from happening again in this new future timeline.
To develop Anakin's character, start with his disillusionment with the Jedi. He believes they have failed him, so his turn to the dark side seems like a way to gain the power he thinks he needs. In the Mustafar part of the fanfic, he can be shown as more impulsive and less in control of his emotions. Obi - Wan, on the other hand, can be developed as a character who is still holding onto hope that Anakin can be redeemed, but is also ready to do what is necessary to protect the galaxy.
One common theme is loyalty. Both Obi - Wan and Padme are loyal to the Republic, and in fanfics, this loyalty often plays a big part in their relationship. Another theme is sacrifice. They are both characters who are willing to sacrifice for the greater good, and this is often explored in how they interact with each other.
In the future setting, they could face new challenges that force them to reevaluate their relationship. For example, if they are in a situation where they have to choose between different factions in the future. This might cause some tension initially, but as they work through it, they learn to trust each other more deeply. They might also start to see each other in a new light, as they both grow and adapt to this new future environment.
There's the theme of trust. In a lot of Anakin and Obi - Wan fanfic, their trust in each other is either strengthened or tested. Another theme is the idea of destiny. How Anakin's destiny as the Chosen One and Obi - Wan's role in it are explored. And the theme of sacrifice. Fanfics might show how they are willing to sacrifice for each other, which is a big part of their relationship.