We'll see how they implement it, but that's one way they could go about using other roles: factions. Not in classic rpg sense, as you get to join officially and then swiftly go through ranks, but more as exceptional hire and part of extended questline.
Sort of designed to mimic the "feel" of being a cop/nomad/rockerboy/media as closely as possible...corporate and fixer would be a lot more difficult, I think.
My dream questline here, really, is joining an anarchy rock band, Streets of Fire style of urban fable drama, at the end you see V singing in front of the crowd...or you end up completely humiliating yourself.
For a cop, even in Night City, you'd have to have a pretty clean record.
The games were far less expensive then. Compare Fallout 1 vs. 2, Gothic and it's sequel, etc: they literally reuse everything from animations, models, UI, etc.
I see no problem with it, especially with content that is more up to personal taste. DLCs, even smaller ones, can add a lot to the game, and developers can be more creative this way. And with something of this scope/complexity, there is absolutely no way they could realize
every idea for the game.
With emphasis on "how to do it":
That's basically what we can realistically aim for and what is the new focus of this suggestion - instead of "classic factions" we get something more to a "as close as possible" approach, but instead of "exceptional hire or select missions" (nice in itself, no doubt!) I think it should open up general opportunities from that group or faction.
E.g. let's say we help the Maelstrom gang somehow, are on their good side. Instead of JUST them later not hindering us or them actively supporting us in main missions or randomly on the street, I think it should also open up something like quest or side quest opportunities with the groups that you have a good rep with. Simpler: instead of changing reactions in main quests OR randomly on the street, side quests and perks should open up so I can go to any Maelstrom gang hideout and get missions and trade with them, or do other social activities that might be added like play some mini-game.
If this will be in the game, then we can basically also look forward to NCPD content. There's surely always wanted criminals or investigations and you could help with the right rep. And get perks in addition.
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A more concrete suggestion - MAXTAC as potential mission giver or employer
Another more concrete suggestion on how you could partake in police content as V and have it realistic in terms of game integration - so you don't have to write tickets as beat or traffic cop:
http://cyberpunk.wikia.com/wiki/MAX-TAC
"They walk like us, talk like us. MAX-TAC are just criminals with a badge"
— Anonymous Punk, Protect & Serve
MAX-TAC might be a realistic employer for V. Because the quote already implies they are perhaps somewhat different in mind and approach. Not your average beat cop. Lethal missions, perhaps you don't get old in this job. Something where V could fit in as supporter or even something like a part-time member. And you do not have to change the cyberpunky personality V might come with if the quote above holds true.
MAX-TAC or Psychosquad will likely definitely play a role in the game and you will likely interact with them somehow already. Interview snippets I remember suggest you yourself cannot go full Cyberpsycho, it would interfere with aspects of the plot or so. So if you're not turning into a target for them, you might still see them or perhaps do missions for them. I assume we already see them very briefly in the gameplay demo.
1) They're mentioned at the crime scene by the NCPD techs
2) We can possibly see what could be a member of MAX-TAC / Psychosquad in the shard Dexter gives us, where we see the Maelstrom gang leader in what I think are cuffs, with a cop next to him and that isn't the usual beat cop attire he or she is wearing. Considering Maelstrom deals with enhancing themselves it could be possible it's a MAX-TAC officer.
The more I think about it, the more it could be fitting employer-wise, as pseudo gun-for-hire / quasi-member maybe. Ironically it would also underline the very first trailer we see with the shootout / massacre where the Cyberlady becomes a MAX-TAC member. From criminal / wanted person to MAX-TAC basically.
I need to end this post eventually so I'll close with this: Basically, MAX-TAC might be fitting as quest-giver and kind of "joinable sub-group" (for perks, 'radiant' quests, etc) as you are not a beat cop, do not have to go to some academy, write tickets. You get search and destroy-like combat missions, can still stay true to V personality wise and ... it's just do-able in the overall plot frame.
They could call you or you could go to them if you want missions involving crazy cyberpsychos and could, otherwise, pursue the main plot. You would not have to drive around and hand out tickets all day or search missing grandmas. Or deal with theft.
And I, as player, would be satisfied if "joining" (I'll use that word carefully now) MAX-TAC would be possible and if you could run around in their gear and do missions for them at your pace or whenever they are available to you.