See, I'd say Tracy Scoggins, Andrea Thompson etc are main cast.
I'd imagine the Telepath War is near the top of the possible stories and might best suit the available cast members, but how do we know it's only one story? Maybe there are several stories, a bit like Lost Tales, there could be one set during the Shadow War with Ivanova and Marcus, for example, if JMS wanted to include Jason Carter.
Well, a year of speculation is going to be torturous, but presumably it's going to be a fairly big project if it's going to debut at SDCC. Hopefully we'll get more details before then.
Trick would be to know "where" and "when" the story was set. Telepath War would be AMAZING, but at present the only information (from posted photos and such) appears to be that Patricia Tallman, Bill Mumy, Claudia Christian, Bruce Boxleitner, and Peter Jurasik are involved. If it were voice-only work, other actors such as Tracy Scoggins, Andrea Thompson and Walter Koenig could have Found a Recording Booth closer to where they lived without having to to travel.
If I recall correctly, Lennier and Lyta both "died off-screen" during the Telepath War, so unless it's a flashback scene, would have to be before that point at least. Then again, Lyta could have faked her death as...
Telepath, and Lennier could have done the same to continue serving Delenn in secret, etc. Anything is possible.
Story might be related to one of the three trilogy novels (Centauri, Techo-Mage, Telepath) or the Drakh War or TMOS, or possibly events spoken-of-but-not-previously-shown such as In The Beginning. Could also be Something
EDIT: How about... Something to do with the Drakh Plague, and the "cure" turns out to be...
But on reflection, if the story takes place after Season 5 then Londo would be under the influence of a Keeper, which might complicate matters as he'd be both an ally and unwilling-enemy.