Hmm, ok yes, I believe that info over the info on Wikipedia. Still, I imagine TNT preferred to have telemovies made that didn't tie in closely with the series being shown on PTEN.
The PTEN to TNT changeover was between seasons 4 and 5. The SE of "The Gathering," "In the Beginning," and "Thirdspace" were all made to boost interest in B5 on the new network, along with daily reruns. I have no idea what the business rationale was for commissioning "River of Souls," maybe just that they had the third movie left on the initial contract and JMS had to make
something, but there you go.
As for a better premise for that movie's slot (and, while we're at it, maybe we could think of an alternate "Thirdspace"), it does depend a bit on what the behind-the-scenes goals were. I remember one of the things JMS mentioned at the time is that he wanted to give the cast who hadn't been in the other two TNT movies a chance to shine (specifically mentioning Corwin), and there's also a question of how much budget there was for redressing sets. One idea would be to show what Lochley was doing during the Civil War, get a better sense of what was going on on the other side or the places Sheridan's march to the sea passed by. You'd lose the chance to show the rest of the cast, though. Well, maybe not. You could have a framing story set in season 5 or "season 6" with Garibaldi, Zack, and Corwin, something like BSG's "Razor."
Expanding our potential scope a bit, a movie covering, essentially, the third book of the Centauri Prime trilogy and covering everything that happened with Londo, G'Kar, the Drakh, David's Keeper, and everything else would probably have made a lot of people happy. And speaking of stuff that was done in prose, the comic where the crew discovers the thousand-year-old wreck of Babylon 4 would've been a lot better than Thirdspace, but I suppose a roadblock in front of that would be Michael O'Hare's availability for the Valen flashbacks.
Still, this is all speculation and nonsense. None of us were there, we don't know what was going on or what the requirements involved were. It's easy to say something more interesting could've been done, but it's hard to know.