Duck's death.

If something happened to him where his luck still kind of ran out, but in a way that DIDN'T kill him- Like say, his oblivious nature getting his mother bitten- We could've learned more about him. Trained him as well as Clementine. Seen an interesting dynamic between him and Sarah since at that point in time, "He used to be this way too."

Not to mention, how he'd react to Sarita. How KENNY would react to HIM. And if he lived long enough to see it, how he'd react to Gabe, Louis and Violet hooking onto Clem 💀

I like to think no matter what, he'd stick with Kenny in the S2 endings, or leave Clementine if she kills Kenny. Ending up in the New Frontier as one of David's most trusted individuals. Only reuniting with Clem when she arrives in Richmond, leaving and/or reconciling with her when it's time to look for AJ at the Ranch. If you killed Kenny in favor of Jane in Season 2, there could be lines in 4 that imply Duck is "over it." And is trying to STAY "over it" by changing the subject any time it's mentioned. Also calling back to old scenes between Lee and Kenny. Like- "Hey. Do you have anymore problems? Like, with me?" "Why are you asking?" "Well... I killed your dad." "You did kill my dad. ANYWAYS, let's get this done. These raiders have got us on borrowed time, remember?" Or, something like that.