There are a million articles on the final episode with various theories and notes about how it ties in with other episodes. Some of the more interesting ones:
1) In the final scene where Kim visits Jimmy in the prison holding cell, the scene is shot in black and white but when Kim lights the cigarette to share the flame is in color. Supposedly symbolizing a renewal of their relationship. When Jimmy takes a drag on the cig, the tip is also in color.
2) After Jimmy fires his final "finger gun" salute as Kim walks away for the final time, Kim doesn't return it. But if you look at her hand at her side, she is making a gun-like gesture. Theory is that a return of the finger gun would be reverting back to old form.
3) The copy of "The Time Machine" that Chuck carries off in the final episode ties in with the recurring theme of "regret" that comes up several times in the final episode about revisiting the past. That same book is shown being carried away by the authorities in the first episode of season six after raiding Saul's home when its significance is unclear.
4) The appearance of Chuck, Mike, and Walter in the final episode (all dead by that point) is supposed to be analogous to the three ghosts from Dickens' A Christmas Carol.
5) Saul requests mint chocolate ice cream as part of his plea deal with the authorities. In an earlier season, Jimmy is shown eating such an ice cream cone when he is abducted by Nacho to represent Lalo. The beginning of things coming unraveled for Jimmy. The cone falls to the ground and is soon occupied by a hoard of ants including one who briefly climbs to the top before being overwhelmed. Supposedly representing the fates with some coming out on top but then being dragged down again. The color green may also represent "money" and how it leads Jimmy down the path to ruin.
6) Gene/Saul's last moment of freedom is in a garbage dumpster. In an earlier season, Jimmy climbs into a garbage dumpster to retrieve some documents that proved important in a class action lawsuit kicking off his first major success as a lawyer. Coming full circle.