you read the title, I'm planning a Spelljammer campaign and I kinda want to add the fantasy Borg.

any ideas?

Have a look at Modrons, they are the closest that D&D has to the Borg. Though the art is awful for them, and they are much less hostile than the Borg. I personally did a Borg-ish villain as a Prime of the Modrons that somehow ended up on the material plane or became interested in humans / humanoids and then went nuts about how chaotic humans were/are and decided all humans must be brought under their control in order to restore order. - Sort of aping the villain motivation from World's End mixed with Borg and D&D.

Unfortunately there really aren't any good statblocks to use, I ended up Homebrewing all the enemies. I based it on the same sort of design as Mindflayer colonies with the "Prime" as the elder brain, then with spider-like robots for the Mind Devourers, more people-sized robots for the Mindflayers, and then a few bigger weirder ones to mimic the Alhoons, Brain Dragons, Mindwitness, Urophion etc... But whereas the Mindflayers are mostly physically weak & rely on stunning/controlling their enemies with their psionics, I made them heavily armoured with more mechanical themed abilities like lightning, grease, and glue based effects.

You might also want to look at the Slaads as those are another "spread by infecting people" monster that comes in a variety of forms.

I’ll give a +1 to modrons.

Also mind flayers: they have a hive mind, fly through space and turn other creatures into themselves. 

As a slightly odder borg option, how about Oozes and Slimes and go for a Founders/Dominion-esque flavour. There is some lore about Juiblex being able to take control of any ooze/slimne/jelly anywhere at any time and so maybe he is using Oblex of various types to infiltrate various planes of exsitence, you could even burrow the intellect devourer and make them psychic gray oozes that take over mortal bodies instead of converting thus allowing them to infiltrate various places.

If it were me, I would design a borg template like the half-dragon template that I could then overlay on any humanoid, fey, giant, beast, monstrosity or other creature I wanted. That would better represent the borg in general, and it would allow for a nigh limitless and utterly unpredictable variety of things to throw at the party.

First thing -- Player Agency: consider how borgifying a PC will play out. Not saying "don't do it", but players can be possessive of their characters. It should not be an easy thing for you to do to them; it should be on the same order of difficulty as killing them.

Second thing -- Themes: Which parts of the Borg do you find most interesting? You don't have to use it all. Assimilation? Group/hive mind? (those two aren't the same. As I understand it, original tv show Borg was a group mind. Movie and later was a hive.) Implacable expansionism? Adaptability?

Third thing -- Details: Magic, tech, biological, or other? Visible or invisible changes? Reversible or not? Can their communication be blocked? What happens to the node if it's cut off?

Fourth thing -- Players are Annoying: What are you going to do if one or more players say "that's cool! I wanna join!"?

Also - (sorry, but I just have to say this) - the guy on the cereal box only has three gold stripes on his epaulettes. Not four.

He's Commander Crunch.

I'll call him Captain when he gets that fourth stripe. 

Sorry. 

Quote from AnzioFaro >>

Also - (sorry, but I just have to say this) - the guy on the cereal box only has three gold stripes on his epaulettes. Not four.

He's Commander Crunch.

I'll call him Captain when he gets that fourth stripe. 

Sorry. 

Depends where he's serving. I know in the British Navy (and I presume the US Navy) captain can be used to refer to any officer in command of a vessel, regardless of rank held

my current idea: The Magav. each Magav "ship" is a massive ooze who floats through space. the ooze is filled with various partially dissolved skeletons. when the Magav "ship" spots a spelljamming ship, they extend a peudopod down to deck, and humanoid oozes step out of the peudopod.

usually I just use the plasmoid statblock, but I have a template for special cases.

when the Magav are in "ship" form their minds meld together so they have kind a group mind.

they capture, not kill. then they bring their captives to an abandoned moon, with a pit of grey slime. they lower the captives into the pit and the captives basically are on the receiving end of the intellect devouerer's create thrall thing. if they fail their save, a section of the grey slime enters their skull and devourers their brain, mucsles, organs, nerves, skin, and last of all, bones, and at the end of that process its a new Magav!

anything I should add?