One of the things I wonder about the new B5 is the "five year plan."
The original show famously had a 5 year plot outline with a beginning, middle and end. This was great in some ways, but bad in others. It had a pretty good ending, but issues with actor availability and syndication meant that they had to change course at several points during its run. They had to force an ending onto Season 4, for instance, because they didn't think they were getting Season 5. Then they did get Season 5, and had to kind of stretch out the story they had left. Then by the end of season 5, when things were going really well for them, people were like "Why are we ending this?"
So far, the new show has not said "5 year plan" and I wonder if that's not an accident.
I think this time around they probably have a vague outline for roughly five seasons. But I would expect the show to be written more as a set of one-season arcs, with the last one being the finale arc, but with the ability to stretch to six or seven seasons if they are doing great by season 3.
And that's assuming we are getting 20+ episodes in a season. If they shoot it in seasons of 8-12 episodes like Westworld or Game of Thrones, all bets are off.