Things that *could* show up (leaving in some ambiguity just to keep some
surprises):

The start of the telepath war
The start of drakh war
Londo's fate on Centauri Prime
The first year trying to make the Interstellar Alliance work
Inter-faction fighting among the Alliance members
The development of Mars as an independent state
The legacy of William Edgars' black projects
Fallout from the civil war, and the feelings about it
How Lennier, Sheridan and Delenn will get along now

How's that just for starters?

jms