Just read the PC Gamer interview, even more excited than I was.
The scope of the affinities and the philosophy behind them is fascinating. Supremacy definitely sounds the coolest to me, that transhumanism idea of leaving the frail human body behind, stuff like that. Harmony has interesting implications as well, being able to use alien breeds against your enemies, even bio engineering your own alien troops. I'm glad they've brought the unit customisation back as well.
It does sound like the team is dealing with pressure from two camps regarding how complex/accessible to make the game. One contradictory thing I picked up on was that they want the game to be much more replayable and the whole custom start thing is part of that. But the monsters on the different types of planet will be the same because they don't want new players to have to learn the rules again when they start a new game.
One last thing though, it really doesn't sound like it matters what faction you choose. You have a lot of freedom to pursue your own in game identity, so beyond perhaps boosts to different traits and a unique unit/building, it seems like the factions are there for the AI's benefit. Like, it won't matter whether you play as Brasilia or the American Reclamation Corporation but it will affect the AI because of the various flavours written into those factions.