One such arc would allow Elly to react to the reality of a long-held wish of hers. As we saw during Phil's single days, Elly yearned for the day when he was married and had children so she could delight in his suffering and tell him that he's finally paying for being born and stealing her childhood and all the other insane bullshit she thinks because she can't and won't get it through her thick skull that her childhood wasn't the hell-world of misery and oppression she wants it to be. Actually having Phil and Georgia become parents would allow Lynn to do the cute kid strips she thinks are obligatory and would give Elly something to do with her time: alternate between being a hypocritical jerk denying that she's talking smack about how incompetent Georgia is as a mother and moaning about how for some cruel reason that isn't "her being terrible at this parenting lark", Georgia and Phil seem to be doing better as parents and also, their kids are polite and respectful.
To see this would, however, require Lynn to use her imagination so is something of a non-starter. It took years for Cathy Guisewite to convince her to create April. It would take forever for someone to convince her that April should be Phil and Georgia's child.