In case you didn't know or if you did where they would be might limit them. Spoilers, so if you don't know what happened and don't want to know, then don't read.
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- Kyle or Takashima after The Gathering
Recalled to Earth, so useful there. Takashima was originally going to be the mole who shot Garibaldi, so that could be interesting to weave in. Dr. Kyle saw Kosh, presumably as an angel, and was never the same. So that could be useful for players investigating the Vorlons, before Kosh is outed at the end of S2.
- Lyta during season 1 & 2
Lyta was recalled for seeing Kosh, presumably as an angel. After being investigated by Psi-Corp, and the Corp not learning much, Lyta goes rogue. Eventually ends up on Mars working with the resistance, with some of the resistance not knowing she was a Teep. May have been involved with the Rangers. After outing Talia, she goes to Vorlon space and shows back up on B5 in "Passing Through Gethsemane".
- Sheridan (and the Agamemnon) before season 2
Definetly useful for players who run across an Omega Destroyer. He was at Centauri Prime when Anna Sheridan left for the rim on the Icarus.
- Na'Toth after her disappearance
She disappears off Narn and ends up under the palace. IIRC, it is implied she was there since just after the War of Retribution. But this is easily changed to suit the campaign.
- Talia after season 2 (Bester says she's dead but who believes him anyway?)
Jms pretty much confirms that Talia was disected, but who really believes Bester anyways. It was shown in "The Corps is the Mother, The Corps is the Father", that multiple personalities in the same person could have varying Psi-levels, so the new Talia would be a P5. It also makes it reasonable that she would be disected if Psi-Corp ever found out that the original Talia was augmented by Jason Ironheart.
- Marcus before season 3
He ran a mining station that his family owned on Arissa. His brother showed up as a Ranger and was killed in a Shadow attack not long after. With the mining station destroyed, Marcus goes to Minbar to become a Ranger in early season 2 as he meets Sinclair. IIRC, in "To Dream in the City of Sorrows", Marcus' first assignment was to help Sinclair stop the Shadows in sector 14. That could leave some time before Zagros VII, but not much.
- Lochley before season 5
She may have also be assigned to an Omega Destroyer, but I'm not sure. Her time is pretty open, except right after graduating the academy.
- Ivoneva after season 4
Ivanova is assigned to the Titans, a Warlock class advanced destroyer. She shows up again before "The Paragon of Animals" to have the Titans cleaned. Very useful for intrigue for players to find out what was going on in Bay 13 and what happened on the Titans when it went beserk.
I'm probably forgetting a few…
Lennier after "Objects at Rest". He's a rogue Ranger with a Nial. What happens to him? Does he work with the Rangers? Does he later die as implied by Delenn's face and manner in "Sleeping in Light".
Lt. Corwin, Cpt. Lochley, Ta'Lon, Ambassdor Vir, Tessa Holloran and Zack Allen on B5 after "Objects at Rest".
Tessa Holloran before "Racing Mars". From "Endgame" to "Objects in Motion" in the newly freed Mars government.
G'Kar after "Wheel of Fire" and before he kills Londo on Centauri Prime a few years before "Sleeping in Light".
Lyta after "Wheel of Fire" when she leaves with G'Kar. She evidently dies in the Teep war. Garibaldi is involved in the teep war.
Londo is pretty well described in the Centauri trilogy "Legions of Fire." It also covers Galen, Kale and a few other technomages. Also covers Vir, Timov and Mariel.
Don't forget Lord Refa in between his visits to Londo.
Lt. Matheson as a teep before the Teep war or as an Earthforce officer after the war.
Cpt. Gideon before being assigned to the Excalibur.
Dureena before "A Call to Arms".
Dr. Chambers and/or her twin sister who is in Earthforce before Crusade.
Max Eilerson before Crusade.
Galen after "A Call to Arms". He was pretty busy and his time accounted for in the "Passing of the Technomages" trilogy before "A Call to Arms."
Alwyn after "The Geometry of Shadows" and before "The Long Road". Also Alwyn after "The Long Road."
Any others that I missed, LBH?
I also thought it would be very chilling for characters to be confronted with a shadow ship during season 4 and learn that its CPU is actually Keffer. Especially if they knew him during season 2.
That could be cool.
Kizarvexis