Yes, but only if they limit it enough in order to not overwhelm themselves with work to do it in the first place.
So the "limitations" of a Subway system that would be more feasible to work on without requiring 4 years to finish would consist of something along the lines of:
- Greatly limit the actual number of stations the player can enter and exit from, I.E. don't fill up the city map with 30+ Subway stations. The low number of fleshed-out, worked-on, properly-developed stations would in the order of 4 or 5 total. Those 4 or 5 stations would be placed strategically across the game map in order to access the main cardinal locations, something general like one station for the Northern portion of the map, one for the South, one of the East, another for the West, and then maybe one in the Center area.
- Physically, seen from street level as you walk around you would still see the Subway train actually stopping by more stations for immersion purposes, but you as a player wouldn't be able to stop there because of course the devs wouldn't have time or resources to actually flesh out the 'real' number of Subway stations that Night City "should" have.
- There would also need to be limitations with the type of interactivity the 4 or 5 stations they'd make would allow the player to do. So you wouldn't need to make things too complicated for the sake of making things more complex. For example, entering the Northern station wouldn't require you to buy a ticket (physically or in-game digitally either), and you wouldn't need to wait for A.I. NPCs finish their waiting line in order to enter the train when it does stop and open its doors (I.E. no need to NPC physical collisions near or onboard the train to avoid glitches and NPCs falling off or clipping through the train doors, etc).
- And once inside the train, as it moves and heads to the next station that you - the player - CAN stop to, the train COULD stop to perhaps another station in between but you wouldn't be able to disembark there because that station is for "A.I. Only", and wouldn't be fully fleshed out enough to bother going there (nor would you be able to physically navigate in and out of such a station anyway). This would only be OPTIONAL on the part of the developers to work on such "in between" stations for added immersion, but it would require more time to code the A.I. in order to simulate the need of A.I. passengers to disembark to that station, and others to get on. I would completely understand even such stations wouldn't be worked on at all, and we'd only get a grand total of 4 or 5 and that's it.
On the other hand, if they do limit themselves in that fashion, they WOULD be able to properly flesh-out, work on and polish REALLY unique stations that each have their own flair and tone, and mood according to the general area they deserve the population for. It would be a lot more pleasant to visit and 'explore' those stations even if there would be just 4 or maybe 5 of them, instead of having something like 20 to 30+ stations but each of them having just one layer of work and each of them in the end looking too similar with very little to distinguish from each others.
If they COULD actually organize their work 'frame' on such a system ahead of time, to ensure that they don't overwhelm themselves with something overly ambitious that can be done in maybe 1 and a half year or so then I think it'd be well within the realms of possibilities for a sizable DLC in the future.