Here's some ideas:
Well, what was it that separated Heero and Relena in the first place? You could go on the angle that he married Noin out of a sort of revenge for perhaps pain Relena caused him? Did Relena betray him in some way? You could consider that option.
Maybe he married Noin because he really loved Relena, but he felt he didn't deserve her (somewhat cliche). However, the twist would be that he married Noin because she was close and comforting to Relena, and therefore he developed warm feelings for her. In other words, she was as close to Relena as he would allow himself to be, and loved Relena
through Noin.
You know what would be really interesting? What if Heero was schizophrenic and he really thought Noin was Relena whenever he was alone with Noin? Schizophrenia can be a sudden condition that can develop at any stage in life, especially when exposed to stressful environments and circumstances as Heero was his whole life. Perhaps a conflict he had with Relena sent him over the deep end or something that
he did to Relena had that effect on him.
Just a few suggestions I thought you might like.
And by the way, this was very sad. I normally don't read stories where Heero and Relena are paired off with other people permanently, but the emotion in this was raw and very convincing. In short, it was a wonderful story!
