In his ideal world, his mother never died. She never got weaker and weaker until even standing was too much effort for her. Here, she is energetic, strong, and always happy. She enjoys cooking their favorite dishes and sometimes she teaches him how to make one; just because he likes learning and she likes teaching. Sometimes she will stare at him strangely for a moment and then hug him, fussing about how much her little boy has grown.
And she never, ever, stares sadly at the window.
In his ideal world, his brother has never had to pay for his mistakes. Except when Ed pisses Winry so much that only Alphonse can get near her without suffering mayor concussions. The rest of the time they act like they always have, competing against each other the way only brothers can. It doesn't matter who wins, they never keep count anyway.
Sometimes they race to see who can get to Winry's house faster. Other times they compete to see who can build Nina the biggest snowman. By the time they are done there's no more snow in the yard and the snowmen are starting to melt. Nina still thinks they are pretty and adorns them with rocks and twigs.
Nina's mother always has hot chocolate ready for when they are done playing in the snow. Nina's father is not there and he has never been, but that doesn't matter: fathers are not necessary in paradise, especially when you have older brothers protecting you.
The only father that matters here is Winry's. He and his wife are both very nice and never get mad at Ed for teasing Winry. They are even a little embarrassed when Ed goes to the clinic to heal damage caused by their daughter's temper.
The clinic is always relatively full and is not unusual to find Colonel Mustang quietly chatting with the Rockbells in the kitchen, over a cup of coffee. Mustang has been their friend since the short war on Ishval and he is—even if Ed finds the guy too smug for his own good—a decent man.
Of course, this ideal world does not exist.
Sometimes, when Ed lies awake at night wishing his dreams were more similar to his vision of heaven, he stares at his brother and thinks he would be happy enough with just a small part of that paradise.
If Alphose can have a heaven, then Edward can walk the rest of the way to hell without regrets.