I am going to make this short... I think it would be awesome if the fallen were eventually made a playable race, not at the launch of the game, but somewhere down the line. Its been said a few times that there is some sort of neutral non-hostile fallen faction... so why the heck not?
I am not talking about them being explicitly guardians or anything... but the fallen are the most widespread group in the solar system, and eventually humanity is either going to have to learn to live with them, kick them out, or kill them all.
and I think it would be awesome if we got a truely alien species to play, and got to see a species achieve some measure of peace with humanity.
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What I am most hopeful to happen is for one of the fallen houses to try and basically become subjects of humanity... to not fight against humanity and its allies, but to become one. It would give us a lot of perspective on Fallen culture and maybe even give us a chance to see a group of them reclaim their former glory with a bit of optimism.