Blood Angels and Dark Angels are very appropriate for Eternal Crusade. Obviously, I want the Salamanders added, but I'm willing to be patient. In the lore, the Salamanders are the only Astartes that actually will go out of their way to protect non-Astartes.
Me personally I would like to see the black dragons cause the use distraction tactic and siege from all sides though their man power is limited
now that I look at a Star Map of the Imperium of Man... the chapters most likely to immediately respond include Imperial Fists, Black Templars, Iron Knights, Night Watch, and Legion of Night... Legion of the Damned (Fire Hawks) would most likely be present as well.
Not really... The Space Wolves have almost started wars with the Inquisition to save non-Astartes. I would recommend going to read the short stories (in particular Terra Nullius and Black Pearl). They explain why each of the Chapters are there except the Imperial Fists. Space Wolves- 1st and foremost it is a Space Wolf world so why do you think they wouldn't be there? Ultramarines- After the Green Tide wiped out the Blood Claw training outpost, the Ultrasmurfs showed up and claimed the world No Man's Land and that if they spilled blood for it, it was theirs. Blood Angels- Mephiston the Lord of Death was tracking a fleet from the 13th Black Crusade. They stopped at Arkhona and realized it was Abaddon's Fleet and called out for help to the other 1st founding Chapters (The Imperial Fists were the 1st to respond). Dark Angels- Like true Dark Angel fashion they were 6 moves ahead of everyone else and were already moving their fleets towards Arkhona for some secretive purpose when a lone Inquisitor braved The Rock to ask the Dark Angels for help. This isn't just some random Imperial Death World that's being attacked and called for help from whoever is closest. Arkhona has a deeper purpose to the factions (other than maybe the Orks cuz you know... their Orks). The have announced that they will be adding other Chapters/Factions in the future but that they won't just be reskins that you purchase because that wouldn't be doing the lore justice. For example they said Salamanders won't even be considered until Flamers are in the game.
I think what might also come up is that adding say, a sixth chapter which would most likely have to be a subfaction would bring up the question of if any of the other factions are getting a subfaction. Releasing one for each might be the plan, if that's the case. If I had to make a prediction on the first post-launch chapter being added, I'd have to say the Black Templars.
