What if a new B5 series began, but as a miniseries? After all, B5 itself was, "technically" a miniseries in that it had a set plot ark that would run its course in 5 years, and that would be the end, regardless of ratings/requests.
If a new B5 series ever came about, it should have a solid plot-ark, behaps one that even extends over only one- or two- seasons. Once people realize what it means to have a "big picture" in a SciFi series again, interest in SciFi will once again increase, and we'll see the road paved for more B5.
Additionally: The show MUST be run on network television, cable and premium channels have too limited a viewership to begin with, and will never pull the numbers required. Remember, Babylon 5 essentially got 3 more seasons out of Season 1, which contains worse acting than many highschool productions. People will overlook shoddy acting and unbelievable-technology (read: the "alien device" in S1, used again in S2 and S4) if you tell a good story that draws viewers in. Since any "good story" has a defined beginning/middle/end, I think that a mini-series would be a good way to pull this off.
(Don't laugh at the idea of a season-long miniseries, remember, 24 was originally billed as a miniseries)