It might be an unpopular view but I don't want Aziraphale and Crowley's relationship to have been part of the ineffable plan. I don't want their love for each other to have been just another chess piece on God's board, destined, since their creation to fall in love. They deserve better than that.
If anything, I want their love to be in spite of the plan. For, just like their sense of right and wrong, their emotional growth together to be hard fought and all their own. I love the idea that it flies in the face of what is expected from an angel and a demon.
I don't want them to love each other because they are supposed to. That would be so diminutive. I want Aziraphale to love Crowley because he's Crowley and Crowley to love Aziraphale because he's Aziraphale. Divine plans and prophecies be damned. Being part of the ineffable plan would erase the agency and beauty from it. Their love is theirs.