You can only date your co-workers in Starfield, decrees Todd Howard
Only the four main Constellation companions will have fully fleshed-out romances.
In an interview with Kinda Funny Games, Todd Howard—director of Bethesda's upcoming Starfield—mentioned that there's a limit to the options of long-term, fully developed romances you can pursue.
An audience question posed by Courtney asked: "Are the Constellation companions the only ones that are able to be romanced?" to which Todd replied: "The four main Constellation [NPCs] are the ones that support full questlines for them, and romance."
This is something of a relief, considering I—and others at PC Gamer—haven't been too jazzed about smooching with Bethesda's usual plastic-faced cast. Still, that long-form romance is only possible with four whole characters in a game with "over 1,000 planets" is wild to me. Apparently, even in a future where you can travel between planets in minutes, a long-distance relationship is still really difficult.
Still, anything's an upgrade over Skyrim's bizarre marriage system, whereupon being wed to your betrothed they would occupy your home like a cardboard cutout cursed with the gift of life. I imagine the limited scope is so the dev team can focus on more fleshed-out romances like the ones in Mass Effect—though as someone who has dated both Garrus and Tali on different playthroughs, Bethesda has a high bar to meet.
That's not to say your crew itself isn't customisable. Another question probed whether they could replace their entire staff with robots, to which Todd gave an uncertain "Technically, yes," while consulting his mind palace. In fairness: Starfield's a big game, so it's understandable that he needed to take a second to think about it.
Whether or not you'll be able to flirt your way across the stars as your own fully-customisable Han Solo is another thing entirely, which didn't come up. I can only assume there'll be some fade-to-black encounters for anyone who can stomach the Bethesda uncanny valley, though it's a shame knowing my spacer's romantically fickle nature will be down to limitations rather than a choice I made for their character.