The new silver hand came mostly from the argent dawn, which was neutral and inclusive. Same goes for cenarion circle, ebon blade, etc.
We, the players, go to war. Lorewise a few individuals from our order halls also go to war, but most of the orders stick around.
Malfurion is one of the leaders of the cenarion circle and yet he is one of two NE leaders, and that works because he has different tasks with each organization, and the night elf phylosophy is similar to druidic. Same goes for shaman, silver hand, and - presumably most of the other orders.
They still have a purpose to fulfull although some smaller than the others.
I could see Turalyon leading an Alliance group that takes over Tyr's Hand, and trying to get the Silver Hand on board, seeing the Horde as evil. And the Brotherhood of the Light might agree, so Korfax leads them to join Turalyon, while you have the more moderate Maxwell Tyrosus continue to lead the more moderate crusaders in continuing their efforts against the Scourge in Stratholme. But maybe with the Alliance taking Tirisfal Glades, leaving Andorhal pretty much defenseless, they take that too, and the Argent Crusade just declares themselves Alliance there, but are hesitant to actively help against the Horde. Sylvanas has proven she doesn't care about them when she kidnapped and killed Nathanos' brother. So maybe she does something to them too, and they join the Alliance.
I would like to see Tyr's Hand and New Avalon and Havenshire cleaned up and used again, it's one of my favorite areas in the game. Turalyon being Alliance is fine, I guess, but he really should be slightly concerned about the state of Lordaeron, and I think it'd fit a little if he were leading efforts in Tyr's Hand.
Though, I dunno just how much Blizzard is gonna be doing things in the old world, apart from Teldrassil being burned, Undercity being destroyed, and the little battleground things at Stromgarde and other places. Probably not gonna be actually in the world, probably gonna be something you queue for like battlegrounds.
I hope maybe you can phase out of new Tirisfal and Teldrassil to regular Cata version, for questing, and nostalgia, unless Blizzard is making something really cool to replace them. You can phase out of destroyed Theramore, and WoD Blasted Lands (so the Dark Portal still works to take you to Outland), so it can't be too difficult.
As for the Ebon Blade, I can't see why Darion couldn't be leader again. It was stupid for him to be a little whiny weasel who doesn't have what it takes to be in charge anyway. And if not Darion, it could easily be the Four Horsemen together who lead them. Though, perhaps there would be conflict there. Nazgrim may want to help the Horde, while Trollbane and Whitemane (who is sane now, because apparently coming back as a zombie you previously would have zealously killed an entire village to snuff out for really does wonders for your mental state) may want to help the Alliance. Trollbane definitely should. Whitemane may want to, but yeah, it seems all the notable death knight characters except Nazgrim would lean Alliance, given the choice.
But, perhaps the Ebon Blade will take a backseat and be offscreen in Northrend for this xpac for the most part. Maybe just Trollbane and Nazgrim will be around, one for each faction, while the other DKs are offscreen.
But, given Sylvanas' hostility to both the AC with Nathanos' brother, and the EB with Koltira (she could be perceived as an enemy by Koltira and Thassarian, and all the EB if Blizzard wanted to go the route of them all rallying at hearing that she tortured their brother in death), maybe both factions will join the Alliance? It would be pretty bad for the Horde in Quel'thalas if that were the case. I could see an Alliance Ebon Blade-led Deatholme stronghold and Alliance Argent Crusade/Silver Hand-led Tyr's Hand being a real thorn in their side. And the Tyr's Hand area also allows for a bit of interaction with Kul Tirans, given their access to King's Harbor. I could see a couple of Kul Tiran ships coming in there to help the Alliance in Lordaeron.
Ugh, I wish it were possible from a gameplay perspective for Quel'thalas to join the Alliance, because if Blizzard weren't bound by gameplay, and it were strictly a story, Quel'thalas would have left the Horde a long time ago. Pretty much after WotLK, probably, with the Sunfury back. All they've gotten from the Horde after WotLK is more and more dead elves in the Horde's campaigns. The losses they've suffered are not worth whatever Blood Golem technology they got from Thunder Isle. Plus, it would mean the high elves could return to Quel'thalas and mend fences, something I'm sure many want to see. At least with non-Silver Covenant high elves.
Imo the new Deathlord should be our champion Minerva. ye, i just love her