I'm hoping that at least one of our companions is a synth but doesn't realize it or is hiding it and it is later discovered (by the player if the companion already knows) during a personal quest or something.
My first mod might be an animated Voight-Kampff machine; if I can manage it. Depends if I get the game.
An annoying little android child accompanied by a gigolo bot (Fisto?) and teddy bear in search of the blue fairy after his mommy abandons him.
I want to see Synths, both obvious and not, in various places of society. Some being servants, others being raiders, some even owning their own businesses. It would be interesting to see one as a religious leader of some sort as well. And I'd really like to see some Synths that are self aware working with the Institute, especially if they differentiate between themselves and their peers that have run away.
I'm also more interested to learn what relationships Synths have with Ghouls and Super Mutants. A society of Synths that have essentially enslaved Super Mutants would be an interesting thing to see, and could even be somewhat married to your idea of Synths as raiders.
Same thing as humans do, depends on the self-determining synth.
The British Film Institute's 'http://www.bfi.org.uk/are-you-a-replicant/' test, is fun.
Having watched "Humans" on AMC fairly recently, one variant that might be interesting to see is one based on Niska - a self-aware "synth" that everyone ~thinks~ is "just another dolly" like the synths were intended to be, and so they put her to work in a brothel. She has to play the role to avoid being detected as self-aware but at some point she is faced with a truly vile scumbag and snaps his neck and flees the scene. She later finds an underground club where humans beat on Synths (who of course cannot fight back) and since she passes for human, they let her take a turn.... except she beats the living hell out of the ~humans~ present and tries to free her fellow Synths but they ARE just regular Synths and cannot even grasp the concept of their own freedom.
I'd be there are at least a few Maverick synths that lead their own raider/evil mercenary groups, but I don't think they'll spawn alongside regular raiders or anything like that. Would be great to see a synth slaver, selling human slaves as payback for humans enslaving synths.