I think we'll get introduced to a few characters referenced in Breaking Bad, even if we really never met them or got to know them well in BB. In fact, it's already happening.
When Saul gets kidnapped by Walt & Jesse in ski masks after their initial meeting, Saul starts freaking and yells out "It wasn't me, it was Igancio."
Nacho's real name Ignacio.
Saul starts speaking Spanish thinking his kidnappers were Hispanic. When Jesse demanded that Saul speak English, Saul said "Lalo didn't send you? Oh thank Christ." I don't think Nacho or Lalo were ever mentioned again in Breaking Bad, but we're already getting to know Ignacio, and I bet we meet Lalo soon. Whoever Lalo is, Saul clearly thinks Lalo wants to kill him (& Nacho) and knows he's capable of it. There could be a whole intricate story surrounding Lalo leading up to the BB plot, and even beyond since it clearly had not be resolved at that point in Saul's backstory.
Other characters & places referenced in BB by Saul or Mike (or people they somehow knew):
1. Saul said Spooge (the meth-head who got his head smashed by an ATM) had been his client
2. The guy they hired to portray Heisenberg so Walt wouldn't go to jail after Badger got arrested
3. Gus Fring (Mike clearly worked for Gus already as a p/t henchman. Saul only know of him through Mike)
4. The nail salon used for money laundering (we've already seen it)
5. The laser tag place and Danny, the inside man
6. The hacker who sent hundreds of donations to Walt Jr's fund-raising website
7. Francesca, Saul's secretary
8. Huell
9. Bill Burr's character (whatever his name was)
10. Vacuum repair guy who helps people disappear
11. Exterminator company they use to hide their meth-making operations
At some point, I think we'll have to find out how Saul and/or Mike know all of these characters, even if it's just a line or two of dialogue.