Better Call Saul is likely to include a cameo from Breaking Bad's Walter White (Bryan Cranston), but that would be a mistake. There has been a lot of speculation over potential appearances from Walt and Jesse (Aaron Paul) in the Breaking Bad spin-off, and it seems increasingly possible as Better Call Saul approaches its endgame.
Over its four-season run so far, Better Call Saul has introduced a number of Breaking Bad characters, both main and recurring. It started off with Mike Ehrmantraut (Jonathan Banks), and has since included everyone from Gus Fring (Giancarlo Esposito) to Huell Babineaux (Lavell Maurice Crawford), but the two biggest would undoubtedly be Walt and Jesse, and in particular the former, since he was the focus of Breaking Bad. It's been commonly suggested that Better Call Saul could include - and perhaps even end on - the scene from Breaking Bad with Walt and Jesse knocking on his door, only from the perspective of Saul Goodman (Bob Odenkirk), but while that would service fans, it wouldn't necessarily serve the series.
Better Call Saul is a real tragedy - more so than Breaking Bad - because we know where Jimmy McGill ends up, and seeing the constant conflict between his failed attempts at being good and his successes that come from slippin' up, is what makes his such a poignant character arc. We're seeing how he becomes that sleazebag lawyer we first met in Breaking Bad season 2, but he already existed before his first in-universe appearance, so it's not like that meeting his what the story of Better Call Saul hinges on. At the same time, introducing Walter White would be such a huge deal that it'd risk overshadowing all around it.
Walt, and his alter-ego Heisenberg, is one of the most famous TV characters of all-time. Even if you limit his appearance to a cameo, there's a good chance that's all fans are going to be talking about. This is about the journey of Jimmy into Saul, and since Walt isn't actually a part of that, it's a bit of fan service that carries the possibilities of undercutting Jimmy's arc because we'd be so focused on seeing Walt again, especially if it was the ending of Better Call Saul.
Even more importantly, though, is that Jimmy's story does not end with Saul Goodman. This isn't just that tale, but it's also the journey of Jimmy into Saul into Gene, and perhaps all the way back round again. Those black-and-white scenes in Omaha that start each season feel increasingly pertinent and exciting; it's there, not in Jimmy's current story, where the true mystery lies. We have no idea what's going to happen to Gene, nor who might be coming for him, or how he might reconnect to his past.
It's in this future that Better Call Saul should become more occupied, and where it should find its true ending, because that's the only way you can wrap-up Jimmy's entire story. It needs to go beyond Saul, and thus beyond Walter White and Breaking Bad too, which is easier to do if you don't introduce Walt and have fans thinking about him as the show's endgame approaches. Walt had a fantastic five-season arc of his own, but Better Call Saul doesn't need him in order to finish Jimmy's.