One thing that might make good TV would be an adaptation (miniseries maybe?) of the Technomage Trilogy. Great potential for using effects, and shows about "magic" seem to have some mass appeal--Harry Potter for adults? Only thing is, this may not be new material for a lot of B5 fans (how much of the fanbase has read the books?), but I think it can offer a new "hook" into the B5 universe for those unfamiliar with the original series, as the story is a whole new angle of the Shadow War (only tricky part would be the places where it ties directly into some of the B5 episodes--you'd have to reintroduce characters like Sheridan). Potentially it could be the whole story of Galen and can continue with stuff covered in the Centauri Trilogy as well (as he was also involved there), and maybe some of where Crusade was going--but just the Trilogy itself would be enough, and probably would work better that way. The Technomage Trilogy was a lot darker overall that B5 itself was, and some of the scenes we'd see on Z'Ha'Dum would be pure horror genre--and there sure is plenty of hair-raising action. Think this would fly in a TV market, and could it be done standalone so that those unfamiliar with B5 can get into it? I think of all the books, this trilogy has the best potential for TV.
Although I'd prefer new material for the B5 universe, I could see a miniseries (or series of movies) based on the Technomage Trilogy as potentially a great artistic and cinematographic work that would appeal both to those familiar with it and those not, if done well. And this is from someone who never got into the "fantasy" end of science fiction before (and still don't for the most part), yet could really get into this story.
Edit: this probably doesn't belong in this thread, which is about actual (supposed) news of what JMS might be up to, rather than suggestions of what we'd like to see him do. But it's sounding like the former may be as much a pipe dream as the latter, alas....