I agree about the travels of G'Kar & Lyta. That would have been interesting. We already got a little glimpse of it in one of the magazine short stories.
Granted, it would not have been as interesting on an epic scale as with the original B5 core group, but I still would like to know what that sixth year would have been like with the "torch" having been passed to Lochley, Zack, Corwin, Tessa (Number One) Halloran, Dr. Hobbs, and best of all, Vir & Ta'Lon.
Vir was always a treat. For someone who was just in a few episodes, I came to admire Ta'Lon very quickly, and the way the "torch" was personally passed to him in G'Kar's message was really dramatic, in a quiet sort of way.
This would probably take far longer than just one extra season (if not one extra century), but it would have been interesting to see the growth of the former League worlds. I can't help but wonder if one of the minor races would have risen to be a more "medium" status. Maybe with the Vorlons gone and the Centauri in isolation, maybe two races would have risen to fill the space in the next "Big 5". Despite acting like dufuses at times, I think the Drazi problaby had the best chance at that, with the Brakiri coming in second.
Other things that may have been interesting in a sixth season:
More about Epsilon 3 & The Great Machine
The rebuilding of Narn
Medical discoveries of Dr. Franklin
Another promotion for Lt. Corwin
The birth of David Sheridan
Maybe some big secret left behind on the Markab homeworld
Sharing of technology among Alliance worlds (especially Earth)
More about the new Warlock class ships and the Excalibur project
A reappearance of Captain Susan Ivanova and/or Marcus Cole
Tessa's further development of a Covert Intelligence branch
A romance between Zack Allen & Dr. Lillian Hobbs (okay, maybe not)