A bit late to the party I know; but I'm just now getting caught up on this.
General thoughts: Even though this is by far the most concrete movement we've seen in the franchise in forever, I've been the Charlie Brown to this Lucy assisted kick enough times that I'm not about to get excited until I at least see a trailer. I mean it early days, and there's still a chance that the network might not choose to pick it up after the pilot is made.
As for bringing the surviving cast (I hate how accurate that term is) back for the new show; I'm pretty sure JMS already said as much years ago when he was talking about the possibility of a reboot. IIRC the example he made at the time (presumably hypothetical) would be that Bruce Boxleitner could play the reboot's version of Santiago (or Clark!) And that the rest of the cast would be given similar guest and/or supporting roles.
Following that pattern there's all kinds of fun possibilities to speculated over: they could bring Peter Jurasik back to just straight up play Londo again, only in this version Londo isn't the Ambassador, but the Emperor, filling some similar plot function as Turhan's role. Pat Tallman could be this show's Bester. Claudia Christian could be as version of Dukhat, with Jason Carter as a her Anla'Shok Na in whatever version of events transpires (because why wouldn't you partner those two up again?) One fun and very ironic possibility that comes to mind is having Andrea Thompson back as the show's ever present ISN reporter. Whatever they find for Bill Mumy, let's hope for his sake it's a human, since he was never a fan of the prosthetics...though there's a certain trichological irony to having him be the new Vir. Tracy Scoggins could either fill something like the General Hague role later on down the line, or some weird alien character as I get the feeling she'd be into it.
A bit of an aside, but one thing that is playing at the back of my mind though is the timing of this next to the whole Dr Who thing. I'm not reading anything into it, but I can't help but wonder what JMS's plan would have been had the Beeb gone for it. I mean there's been rumblings about something B5 in the works for a while now (in hindsight the remastering of the show on digital may have been to test the waters in terms of interest) so presumably talks have also been ongoing for a while.
Even JMS isn't crazy enough to run two shows at once, so I have to wonder which one he would have dropped, or if that whole move was some sort of negotiating tactic...though damned if I know how that would work since they said no.