The fifth story in the time-travel saga would involve Count Manzeppi, who appeared at the very end of Airship as a teaser to signal his upcoming involvement. I am not as yet sure what Manzeppi would be after.
One idea would continue the events of the episode The Night of the Feathered Fury and have the chicken with the Philosopher's Stone inside turn up in the present day, thus prompting Manzeppi to begin a new search for it and involve all of the main cast of the time-travel saga in the process.
Some time ago I wrote a random scene where Manzeppi tries to trick Coley into believing that Lafe got greedy, tried to take the chicken, and was turned into gold as Greta supposedly was. Manzeppi ends up with the chicken, while Coley is left reeling with the supposed Lafe statue and trying to deny what's going on. Lafe then stumbles up from a stairwell, where he had been knocked out by one of Manzeppi's men. The statue really is just a statue and always was.
That scene, or something like it, will definitely feature into the story. Another planned scene is for Manzeppi to lure Snakes into a trap and have a rebuilt Deadeye robot threaten to poison him if he doesn't join them. Deadeye manages to break Snakes down and he exclaims that he will join, whereupon he is released and falls to his knees in tears, badly shattered by the threats of being poisoned with the same substance Posey used on him years ago. Snakes really has no intention of joining Manzeppi, however, and only agreed to make the psychological torture stop. When Manzeppi tries to get Snakes to kill Jim and Arte as a sign of loyalty (planning to kill Snakes after Snakes kills them), Snakes turns the gun on Manzeppi instead and Manzeppi has to make a quick getaway.
I have always thought of the Deadeye character as a robot, since in The Night of the Eccentrics Manzeppi says he's a machine and has oil in his veins instead of blood. Deadeye is interesting in that he seems to be able to think and reason like a human, but he is completely cold and cruel and cares nothing for human life. I've wanted to contrast him with Little Pinto and get them into a confrontation, so that will also likely happen at some point.