Okay you know that line in the deathly hallows that Fred says. "When I get married".
Fairly sad, he will never get married he died.
But what if he and Angelina had had an on and off again relationship (if you are familiar with Degrassi, think of Eli and Clare, but with far less drama) and they were together at that point and only George and Charlie knew because Fred didn't want his mum to obsess over his relationships. George would never tell anyone and quite frankly Charlie's thoughts were 'as long as your happy'. And what if he was planning on proposing to her when the war was over? And George was going to help him set things up. Think fireworks and really dramatic but cute proposal, because that's just who Fred is.
And then during the battle of Hogwarts in that last time before Fred and George separate Fred says that he's going to propose next Saturday and he's really excited.
And then he dies and when Fred is lying in front of the Weasleys George crumbles and goes to find Angelina.
He tells her what happened and what Fred had planned.
And she just goes blank. No tears, no pain, she's just empty. Her entire world has gone. She too was planning on proposing because why should it be the male that has to propose, screw gender stereotypes.
And George and Angelina just sit there for a while, amongst the debris, unable to cry anymore because they are just numb. The shock wears off eventually and they rely on each other.
They are together through the nightmares and the guilt, the 'it should have been me'.
And Angelina and George work together in the shop to try and distract themselves. They put on a smile for the customers but in the evening when the shop has closed they just break.
And slowly they begin a relationship. It's hesitant, like it's disrespectful to Fred's memory. But somehow, it's okay as well because they both lost half of themselves when Fred died and together they feel whole again.
It takes about six or seven years to even reach the relationship stage because for years they were too broken from Fred's death. And after those six or seven years it's another two or three years of dating before marriage.
And at the wedding there's an empty seat in the front row for Fred.
(Luna had started this tradition at Ginny and Harry's wedding with a row of empty seats for his parents, Fred, hedwig, Dobby etc).
For two such popular people the wedding was small, and a part of the wedding reception was dedicated to Fred.

I feel like George and Angelina would be a QPR, no romantic or sexual attraction, but two people who deeply rely on each other and care for each other. They adopt Roxanne when she's eight and adopt Fred II when he's six.