I believe that others factions will have a kind of relation with the campaign.
So, what do we know about this ?
We know that HW2 is 2 months later that Halo 5.
We know that Cortana wants to control our galaxy.
We know that Floods has infected Spirit of Fire.
So, I think we can imagine easily what will be others chiefs.
First, I think it is evident that Cortana’s faction will appears, as a Foreruner faction, with the (or a ?! ^^) Warden Eternal as hero.
Secondly, If Flood appears in the SoF, so I think (I hope) the flood faction will be playable, as Ensemble Studio had planned. Don’t know who could be the hero.
Thirdly, I’m sure that Lasky (or other of “new” UNSC faction) will appear. Just because the story is about the Ark. So it’s evident that’s UNSC will participate to the battle. Hero ? Master Chief.
Finally, why couldn’t Swords of Sanghelios appear in HW2 ? A new faction of Covenant to beat ? With Brutes ? Thel 'Vadam absolutely will want to participate to the battle !
What do you think about ?
Sorry for my english, I’m french, be indulgent please.