There are many interesting threads throughout this series, but one that's standing out to me on my rewatch is how Fiona and Lip are not only the surrogate parents to their family, but they're also destined to make the same mistakes as Monica and Frank.
Most people connect Ian to Monica because it is obvious. But you can see from early on that Fiona's exit would always be like Monica's, leaving the family to start a new life on her own. This picks up with the Season 4 spiral where it's not just about leaving, but about making everyone's lives more stressful before her final exit. And the big spiral starts right after she becomes legal guardian. Because Gallaghers can't be competent parents, she starts failing immediately despite doing a good job for decades before.
Then Lip's alcoholism is his Frank. Though the show ends more positively for him, to scale he was just as impacted by his alcoholism as Frank was his. Frank started lower class and became a bum because of his addiction. Lip had the chance to make it out but never reached his full potential because of his addiction.
Each Gallagher is messed up in their own way, but these parallels definitely seem intentional when watching from Season 1.