I'll never forget the dread, or the beautiful simplicity of the Borg. As a child, few things in literature impacted me as much as that idea. It's so plausible that they likely already exist. They solved all of the problems that biological entities need to solve in order to colonize the cosmos. It was terrifying, but as I grew older I realized that thier loss of humanity was a feature of evolution. Horrifying and oddly comforting at the same time.
For a while, the Borg represented my idea of absolute horror - unstoppable, zero goals in common with us, they never negotiate because they're so different. They seriously nerfed them later though, so they kinda lost that vibe.
It's an interesting thought experiment to imagine if the Borg has optimized for aesthetic appearance. Which is to say, if the TNG show runners had made them pretty.
Their hybrid organic/mechanical appearance is obviously calibrated by the show designers to unsettle viewers and produce a negative reaction. See: David Cronenberg
But how would we (TNG viewers) have felt about the Borg if they looked like us... but were just better?
(Voyager sidestepped this by making a pretty Borg explicitly not-Borg. But still had some more interesting exploration of the right- or wrong-ness of Borg philosophy)
The crazy thing about the Borg is people would absolutely sign up for assimilation if it were consensual. The perfect peace and tranquility of abandoning your self to a vast collective, having your entire civilization archived, essential posthuman eternal life (depending on whether the "Borg" really exist as data, just using drones as necessary physical interfaces, which is my own headcanon but I don't think actual canon.) People would absolutely give up individuality for that.
For a species with vast collective intelligence and access to data on countless cultures, they seem to have the worst possible approach to their goals. They assimilate by nanoprobes, after all - they could easily create replicants that resemble a target species and have them blend in, slowly spreading a dormant "infection" that doesn't trigger until it's spread through enough of the population. Just throwing cubes at anything with a heat signature works in a brute force way, but there are more elegant methods.
A more aesthetically pleasing, subtle Borg as a foil to the Federation would be interesting. It frustrates me how a lot of new Trek has been sort of teasing at plot points involving AGI and synthetic life and cybernetics but they never really dive into any of it.
IMHO the early Borg represented a fundamental difference
The Federation: Freedom, independence, self-determination
The Borg: Hegemonic for the sake of it, absolutely opposed to individuality
Even beautiful Borgs would be horrendous, but depending on the context it might discount something from the ways they're different. Borg look like they do because they don't care about their bodies, beauty means nothing to them.
The Federation has very rigid and even conservative stance on body modifications - and of course their whole agenda regarding individual freedom.
Nowadays, my idea of absolute horror is an Outside Context Problem (viz Excession by Banks).
Have you read Vinge's A Fire Upon the Deep? You might enjoy it, if Excession sparked your fancy.
Note: Vinge is more circumspect and less obvious about what's going on, so expects you to infer more than Banks does. Also, would recommend reading Fire without Googling it, as that would spoil one of the nice literary sleights of hand.
