The thing about B5 is that few of its space scenes involve much compositing, since they were all CGI from the start and compositing was hard back then. For that reason, I think a space-effects redo would be unusually feasible for that show, albeit still too expensive for anyone to actually do one.
A harder, if smaller, remastering issue would be that they shot practical effects on video and special effects on film, largely due to that being the cheapest option for both. In standard definition you can't really see the difference, but watching the show today you definitely do.
Sadly, such an approach is impossible for the majority parts of the latter series (DS9, VOY) since their effects were mostly CGI, and rendered directly to standard definition video. Further more, many of the 3D models and scenes have been lost, so it would probably require a total effects re-do from scratch (on top of film stock scans), which is "a very costly proposition with questionable (financial) returns".