Interesting side question: Who would bring in more players to EC? Tau: Anime fanboys Necrons: Live out your Terminator fantasy!
Tau are a non-option due to lack of real FTL and the fact that their empire is a literal booger on the pavement that is the galaxy. Dark Eldar aren't the kind to take and hold planets, same can be said for their space panda brethren but there's allegedly something shiny on the star system. This leaves Necrons as the only real fifth option, they are often the shiny thing, they have a unit for every possible class and more, poor on weapon variety but tougher and more interesting potential mechanics.
Both of which are set in the Ultima Segmentum, where their empire is located. Your point? EDIT: Medusa V, also Ultima Segmentum, even GW respects their lack of FTL in their campaigns.
Quite frankly in terms of lore, IG would make the most sense. I mean, it was an Imperial world, they lost it, must be pretty pissed. The sector in which the game takes place is in segmentum obscurus during 13rth black crusade were a lot of IG regiments are fighting around Cadia with support from other elite regiments of elsewhere in the galaxy. Necron wouldve been awaken by the inhabitants of Arkona back when there was an actual population. They wouldn't have waited for everyone to die to tyranids and for every other entity to come to the planet. Or at least, it was presented in other media that an inhabited world would usually wake up the necrons. (DOW i.e). Tau makes no sense unless they use some plot device like a kind of portal. Adeptus Mechanicus would make sense because of the number of titans. Maybe they have weapons there an even research facilities that they would not want to lose to abandon. Conflict could rise with marines if one side wants to destroy said facilities while the other wants to secure it back. Sister of battle would make sense if they suspect heresy on the planet. So my bet would be any missmatch of sisters, guards, inquisition and adeptus mechanicus first. Dark eldar are highly unlikely because they won't get souls easy there, the "civil" population all died in unknown circumstances. They don't want to die and give their souls to slaneesh so I'm guessing they would avoid a suicide mission going against so many very powerful actors. necrons I think if they were there they would've awaken a long time ago. tau needs an explanation I do not have.
I actually thinks new factions have to be added in pairs, to offset players massively migrating to a new faction to try it out. For me most sense make Necrons and Dark Eldar as first factions. Obviously after 4 main factions are completely done and balanced. Preferably when some sort of campaing map is done and we also can have a big map with vehicles riding around, bikes, etc.
