Better Call Saul season 6 needs to properly explain a line about Lalo Salamanca spoken by Saul Goodman in Breaking Bad season 2, though there's a question over just how it'll do it. Long before the prequel show was planned, Saul dropped a reference to the character who'd later become a villain in his own series. After being kidnapped by Walter White and Jesse Pinkman in Breaking Bad season 2, episode 8, "Better Call Saul" and taken out into the desert, a panic-stricken Saul pleads with his captors before learning they're not who he thinks they are, saying "Lalo didn't send you."
At the time, it was a fairly meaningless line - an example of Breaking Bad's canny word-building and easily believable as a part of Saul's backstory that didn't need to be told. Except, of course, that said backstory became the basis for an entire show, and Better Call Saul has now started building towards explaining what that Lalo line referenced. First introduced in season 4, Lalo grew into a major player in season 5, going up against not only Jimmy McGill and Kim Wexler, but also Mike Ehrmantraut and Gus Fring, positioning himself as the Big Bad of the show.
While Better Call Saul has used the Lalo line as a springboard with which to introduce the character, it hasn't actually explained why Saul references him in Breaking Bad - and it's going to be interesting to see just how it does so. There's still a gap of a few years between where Better Call Saul is and when it's used in the main show, which would suggest that either the timeline will be blurred somewhat, or there'll be a clear time jump. Either way, Lalo needs to be a part of that, and it suggests that he needs to survive whatever happens, or at least Jimmy needs to believe that he does.
If Lalo isn't going to die, then one possible scenario is that he does end up getting sent to prison for the murder of Fred Whalen after a lengthy trial, having already skipped out on bail once. That would mean he's still alive at the time of Breaking Bad, and if it can bring the events closer together in time, then it's possible it isn't long after his imprisonment that Saul is kidnapped - explaining why he's still looking over his shoulder for Lalo at that point, since he would still have power and influence even from a jail cell. While Jimmy (and Kim) could be involved with that, Better Call Saul also needs to explain the other part of the line: "It wasn't me, it was Ignacio," which fans now know refers to Nacho Varga.
Better Call Saul season 6, then, has to set into motion events that transpire close in time to Breaking Bad, which involves something happening to Lalo that could feasibly be blamed on Nacho. Again, the likely scenario there is Jimmy (or someone else) ratting Lalo out and resulting in him being sent down, though the prequel has enough plates spinning that it could find a solid alternative. Nacho has already attempted one hit on Lalo's life, and so it could be connected to another attempt to take out Lalo more permanently, and/or tie into Juan Bolsa's own efforts to disrupt the cartel (which Jimmy has unwittingly factored into).
Saul likely wouldn't give up Mike or Gus because of how powerful they are, but Nacho is an easier fall guy for him to target, so it makes sense he'd use him to cover his own tracks - assuming, that is, Saul really did do something to make Lalo come after him, which would be keeping with his character. With Lalo seemingly the endgame villain of Better Call Saul season 6, then it would sense for all of its storylines to coalesce around him in the final run of episodes, bringing together its disparate elements to take him down and explain Breaking Bad's mysterious line.