I may have mentioned this before, because this was mentioned a tiny bit in old notes that JMS put in one of the script books, but Artifacts from Beyond the Rim goes into far more detail about this. David Sheridan, the son of John and Delenn, was going to travel backwards in time and show up for a few episodes, preaching against Sheridan and the ISA. At first, I thought this was a totally stupid idea, but in this book, it gives an outline for the whole thing, and it's actually kind of interesting.
So this is the idea. A snub-nose White Star, the same type we see Sheridan use in Sleeping in Light, is found floating dead somewhere, life signs are detected, and they go aboard and find a young man on it. He's taken to B5, put in Medlab, and recovers. His memory is messed up, and after a bad incident involving a trader that gets killed, he blames Sheridan for it. In the next episode, David keeps preaching against Sheridan and is gaining a following. Sheridan and Garibaldi watch this and are upset, but Sheridan strongly believes in free speech and feels that he must allow this. Eventually fighting breaks out and while David is detained, Sheridan meets with him and finds that he actually quite likes him and feels a connection, even though David still hates him. David starts dying for some reason, with his followers thinking that Sheridan tried to have him killed. The main characters can't find out why he's dying, this is how the second episode ends.
In the next episode, the final part of this, Franklin is treating David and discovers that he has human and Minbari DNA, and not only that, but finds that Sheridan is the father and Delenn is the mother. On a hunch he checked and found the same time distortion effects that they experienced before. They decide that they have to get him back immediately, and find the temporal distortion field that he came through. He goes back through, and right when he's about to go, he remembers something, a danger ahead, but then he's gone.
At first I thought that this whole thing would totally contradict the rules of time travel and such established earlier (which would really annoy me), but now with most of the context, I think that it doesn't really. Rather than coming from a timeline where Sheridan and Delenn turned into dictators, it seems more likely that David had a big problem with them, and this actually ties in with what's mentioned in The Deconstruction of Falling Stars, when the people a hundred years later mention a major controversy with the son of Sheridan and Delenn. David Sheridan getting a keeper put on him seems to be something he thought up later, and this is also supported by the fact that the notes for Objects at Rest doesn't mention Londo visiting on Minbar, so I think that there was some other incident that David was going to be involved with instead, rather than a Drakh plan with a keeper.
Overall, I think it's better that we didn't get this, but it did turn out more interesting than I thought it would be.