Warning: possible spoilers ahead for Better Call Saul, don't read any further unless you're caught up on Season 2.
As far as spin-offs go, Better Call Saul has done pretty much everything correctly. Following Breaking Bad was never going to be easy, but the origin story of Saul Goodman has carved out its own niche while still feeling firmly entrenched in a world that will eventually see the rise of Heisenberg. Saul has slowly inched towards the Breaking Bad mythos over the last two seasons, and the re-introduction of Giancarlo Esposito's Gus Fring (among other classic characters) will continue that trend in Season 3. Bob Odenkirk elaborated:
The presence of Gus Fring in Season 3 is something that fans of Better Call Saul have expected for quite some time, but Bob Odenkirk's recent comments to The Huffington Post seem to indicate that the show has quite a few tricks left up its sleeve. The impending arrival of Breaking Bad's best villain is going to be a major plot point for Season 3 -- particularly for Mike Ehrmantraut, as it seems to be Gus who told him not to shoot Hector Salamanca. However, other familiar personalities will also make their way back into this work to interact with the proto-Saul Goodman for the very first time.
This development naturally raises very important questions about which characters could show up, and where we may see them. In the primary Better Call Saul storyline (which takes place in 2002) we could easily see characters like Hank Schrader, Steve Gomez, or even the dynamic duo of Badger and Skinny Pete make appearances. However, that's only one aspect of the show. A significant portion of Saul's story has already been devoted to Jimmy's time after Heisenberg's reign of terror, which means Aaron Paul's Jesse Pinkman could show up on his doorstep after escaping from his prison in the Nazis' compound.
The possibilities seem endless at this point because Bob Odenkirk has made it abundantly clear that the Breaking Bad connections are only going to become more pronounced over time. He explained:
All this talk of surprises has us wondering if we will see something come completely out of left field. Better Call Saul has already played with the timeline of this universe quite a bit, so could we potentially see flashbacks to his time working with Walter White? Could Bryan Cranston possibly return this season as the legendary meth cook? It's anyone's guess, so we will just have to wait and see.