'Law & Order' and its spin-offs can be seen as a mini-version of Toobworld. Characters who are long gone from the show might still show up one day even years later. Unless of course, the character died in a storyline, like Detective Sgt. Max Greevey or Detective Lenny Briscoe, or if the character is played by Michael Moriarty.....
Former ADA Paul Robinette came back as a defense lawyer, as did Jamie Ross who is now a judge as seen on 'Law & Order: Trial By Jury'. Detective Mike Logan was banished to Staten Island, came back in his own TV movie, and is now a regular on 'Law & Order: Criminal Intent'. Captain Donald Cragen is working with the 'Special Victims Unit', and
Dr. Olivet even crossed networks to appear on a non-'L&O' show, 'New York Undercover'.
Detective Munch of 'SVU' is a master at this, appearing on three different networks in the most TV show/TV movie crossovers. And his character started out on 'Homicide: Life On The Street'!
Even recurring characters help maintain this stability for the universe - judges and lawyers return often to all three shows.
And now we have another one. Assistant DA Alexandra Cabot had been shot by drug lords and everyone thought she was dead. But in truth she was recovering while in the witness protection system.
She'll be back full-time now according to news reports, as she shepherds a covey of young ADAs in the new 'Law & Order' show, 'Convictions'. (What aggregate term do you use for ADAs?)
Me, I'm hoping that one day we'll see Edie Falco show up on one of the 'Law & Order' shows as her defense lawyer, Sally Bell.