While I am super excited for the game I am quite sad that there will only be six playable factions on release since every game before (Legend 1 and two Space games) had 8 on release. Any reason why this choice was made? I hope they make more DLC though. I love the asymmetric gameplay Endless Legend offers like no other.

I can only speculate, but given that asymmetric gameplay was Endless Legend's strength, I see the reduced factions as a required tradeoff since fewer factions allows you to spend more time/effort creating something unique. You can take the CIV route of many factions with broad overlap in units/gameplay/tech, or the Endless Legend approach of fewer factions that feel very different. With EL2, I think the developers or going for even greater asymmetry, which requires even fewer factions relative to EL1. Already we see factions will have 4 units (5 for the Kin) and each unit can specialize into two different units; this is already far more variety than EL1, and so far I like what I am seeing. If the game does well, I think there will absolutely be DLC, and each DLC will likely introduce one, specialized and unique faction, gradually increasing the faction count overtime. 

It seems like factions in this game will have more units, more building types, more heroes, etc. And that will probably also mean more assets need to be created, which in turn means that expenses and time to make those will be greater for each faction. So even if we are getting only 6, the cost of making them is probably like twice as high as the cost each faction had in EL1.

THAT SAID, I do agree 6 feels a bit underwhelming. HOWEVER, the community manager said in Reddit that they are considering designing a faction with the community. However they are not confirming whether this will be the sixth or a seventh faction, or if work on it will come during the EA version or after the full release. So they are probably tracking community engagement levels, profits, etc. as well as seeing how progress goes on with the main factions in order to decide.

So I think that if you want the game to have a seventh faction, I think you should probably promote the game just enough that they might try it. Remember that Harvesters, The Cult of the Eternal End and the Unfallen were all community-designed factions, so Amplitude does have a track record of doing such things.