With Breaking Bad.
If that's the case, it stands to reason that some fan-favorite characters from Breaking Bad might show up in the final few episodes of Better Call Saul. Especially if these episodes take place during the time in which the two shows' stories converge.
Gale Boetticher
Although Gale has already appeared very briefly in Better Call Saul, many fans are speculating that the character could play a much larger role in the show's final season. In Breaking Bad, Boetticher is the chemist that works alongside Walt in Gus' lab and eventually dies at the hands of a morally conflicted Jesse.
Gus' feud with the cartel is growing stronger and stronger by the episode in BCS' sixth season, so it would make sense for him to get involved with Gale to help with the improvement of his product. Gale is also an integral part in the downfall of Heisenberg's story, so there could be a few shocking secrets to uncover there too.
Tortuga
In Breaking Bad, Tortuga appears briefly as a member of the cartel working for Juan Bolsa. Although the character meets an unfortunate end when his head is cut off and placed upon the back of an explosively-charged turtle, there's nothing to say that Tortuga couldn't appear in Better Call Saul.
The conflict between Gus Fring and the Salamanca Family is a huge aspect of both shows, with Juan Bolsa acting as a sort of mediator between the two. His loyalties lie with the cartel, but he does not share the same hatred for Gus that Hector does. Perhaps he and his men, including Tortuga, could end up playing a bigger part in the conclusion of Better Call Sall.
Andrea Cantillo
Although Andrea has seemingly no connection to any of the main characters in Better Call Saul, she remains one of the most popular and sympathetic characters in the Breaking Bad universe - so a small cameo seems fairly plausible, and it would also make a lot of sense narratively.
Jesse first meets Andrea in a Narcotics Anonymous group, so it's clear that she has some sort of history of drug use. To this end, it's possible that she could come into with the Salamancas, even indirectly, if the show explores her backstory a little more.
Marie Schrader
Hank's wife Marie was originally supposed to make a brief cameo appearance in Better Call Saul several seasons back, but the scene never really came to . Marie works as a nurse at the Radiology Center, so it doesn't seem too unreasonable that her character could appear if any of the main cast found themselves injured or hurt.
Marie has plenty of iconic quotes and memorable moments in Breaking Bad, so the return of her character could be a really amusing and funny one if played right. Her relationship with Hank could also come into play, supposing Hank finds himself involved with the Salamancas again.
Lydia Rodarte-Quayle
Lydia has already appeared in a couple of scenes in Better Call Saul, but her presence is yet to be anywhere near as important as it was in Breaking Bad. She played a huge role in the final season of the show, and her relationship with Gus makes it likely that she'll appear again at the end of Better Call Saul.
Her ambiguous morality left Lydia somewhat of a mystery, though those moments when she veered more towards being a villain made her loved by most fans. She's a fun and complex character, and there's definitely room for her to play a role in the conclusion of Saul's story.
Todd Alquist
Todd plays a huge role in the final seasons of Breaking Bad, particularly regarding Jesse's tragic storyline. But whilst it seems unlikely for this dynamic to be explored in Better Call Saul, there is one other character that could act as a gateway to introduce the character - Saul Goodman.
When Mike hires Todd and his gang to help with an operation led by Walt and Jesse in Breaking Bad's fifth season, it is implied that Todd has been represented by Saul in the past and perhaps even bailed out by him. This is a story that could definitely be explored in the show, perhaps relating to the Salamanca's control of street-level dealings.
Jack Welker
One of the most shocking moments in all of Breaking Bad was Walt's brilliant (but totally evil) plan, which called for the murder of nine of Gus' former employees whilst they were in prison. In order to do this, he employed the help of Todd's mysterious uncle, Jack Welker.
It doesn't seem too unlikely that Jack will appear in Better Call Saul's final season, even if it's only in a brief cameo role. The criminal has so many connections and allies throughout all of New Mexico, that it seems plausible that at least somebody knows him somehow.
Skyler White
Although many fans love to hate her, Skyler White remains one of the most interesting and complex characters in the entire Breaking Bad universe. She interacts with Saul several times throughout the course of the show, so it stands to reason that she could him after the events of Breaking Bad end.
Although he's moved out of New Mexico and assumed a new identity, it's fairly clear that Saul still has unfinished business that he's trying his best to avoid. Skyler could be a really interesting way to bring these problems to light, allowing Saul's journey to reach its definitive end.
Jesse Pinkman
It's already been pretty much confirmed by showrunner Vince Gilligan that Jesse is going to appear in some form during the final season of Breaking Bad - so the only remaining question is: how? The character has seemingly no connection to the criminal world before meeting Walter White, so it seems unlikely he'll come into with Saul during the timeline of Better Call Saul.
Thankfully, fans have come up with two plausible answers. It's possible that the last few episodes of Season 6 will take place during the Breaking Bad timeline, retconning certain moments from Saul's perspective. Jesse could definitely show up there. Or alternatively, Jesse could appear to Saul (or Gene) during the show's later black-and-white flashforwards in Nebraska.
Walter White
Season 6 will mark the definitive end of the Breaking Bad saga, according to writers Gilligan and Gould. With that in mind, it seems unlikely that this entire story will wrap up without Walter White appearing one last time. His death at the end of Breaking Bad makes it unlikely that anything new will be added to his story, but an appearance isn't off the cards.
The internet is full of unpopular opinions about Walter White, so an appearance in Better Call Saul could definitely help add some new information or clear up some questions about his character that fans have held for a long time now.