Now, while the Law Firm is great, it is quite lacking, it is basically just a money-making headquarters (As is the Web Development Agency). I will put 2 sections below as a Simple Revamp (something the developers could probably finish within a couple of betas or the first early accesses) and a Advanced Revamp (Long term development plan).
Simple Revamp:
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Use already existing offices, allow clients to come in and sit in a chair connected to an employee’s desk
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Mailroom Courier - (Employee/furniture) An employee for a law firm could be a Mailroom Courier, who delivers case documents to clients via a Modified Handtruck (Essentially a shopping cart, for a reference image search up “4 sided mesh truck”) + Industrial Printing/Copy Machine + Computer + Courier Station/Desk
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Law Firm Executive (Employee) - Essentially a HR Manager, but only for Law Firms. Only requires a simple desk/computer/chair. This employee will revise all cases (I don’t have an idea on how to implement this feature, you could just for example give a 20% profit increase when you have a Law Firm Executive)
This is all for the Simple Revamp, shouldn’t be too much work to add to the game, while also making the game more immersive/have more content.
Advanced Revamp:
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New building type: Large Office (Building) - This office will be much larger, have many rooms, and a large open space (This building could also be used for other businesses like Web Development/Headquarters and future office types.) Due to the size, however, I would recommend implementing an room distributer/marker, for ex. Paralegal room, Mailroom, Law Firm Exe. Office, etc. (This room marker system would automatically mark the rooms depending on business type, you could take inspiration from Prison Architect, a game with a similar feature)
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Mailroom Courier(s) - (Employee/furniture) An employee for a law firm could be a Mailroom Courier, who delivers case documents to clients via a Modified Handtruck (Essentially a shopping cart, reference image below) + Industrial Printing/Copy Machine + Computer + Courier Station/Desk. This employee(s) would require a specified room, and depending on what the customer capacity is of the office, lets say the customer capacity is 120, and each mailroom worker delivers 30, so you would need 4 mailroom couriers.
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Law Firm Executive (Employee) - Essentially a HR Manager, but only for Law Firms. Only requires a simple desk/computer/chair. This employee will revise all cases and be responsible for Class Action Lawsuits (Only really need this employee for class action lawsuits)
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Paralegal(s) - In real life, Paralegals manage documents for lawyers before a trial, hearing, or meeting. To simplify it in game however, lets say each paralegal manages 5-10 lawyers, depending on Paralegal skill level. Paralegals will require an office (Not one for each, one for all paralegals, unless you want 1 for each) with a desk (Computer, desk, chair), industrial printing/copy machine, and industrial stamping machine (Simply a stamping machine, used in real life for validation).
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Law types - Now, in real life, there are many different types of law. (Elder law, Divorce law, Traffic Ticket law, etc.) But, in game, you could simplify it down to 3-5. Here are some good law types:
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Traffic Ticket law - The least profitable type of law, makes around 100-500 per client. Requires a dedicated office per lawyer, with a desk (chair, computer, desk, and also another chair outside of the desk for the client to sit in). Once the design feature is implemented, design level can increase the income/profit of traffic ticket law to 500-1000.
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Criminal Law - Obviously, law about criminals. You do not need to specify which types of criminals/offenses, and this type is the 2nd worst type. This can receive $2000-5000 per client, depending on Lawyer skill. Requires a dedicated office per lawyer (Chair, computer, desk, and also another chair for the client to sit in). Once the design feature is implemented, design level can increase the income/profit of criminal law to 5000-7000/10000
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General Law - Not a real law type, but for the sake of simulation we must have it. This law type is the lowest paying type, paying $100-300 per client (Can be raised to 800 per client when design feature is implemented). Use the already existing law firm in the game, just requires a desk per lawyer in the open space of the office (Except have a chair so the client can sit).
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Real Estate Law - Deals with real estate, Highest paying law type (Besides Class action/Civil litigation), $20000-50000 per client. This will require a lawyer and a dedicated office for the attorney with a desk (Chair, computer, desk, and also another chair for the client to sit in). Once the design feature is implemented, design level can increase the income/profit of real estate law to $50000-70000
Bonus Law Type - Civil litigation law/Class action lawsuits - A very rare event that can happen when working. It can pay up to $25 million per lawsuit (Starting from $5 million), and it requires at least 15 general lawyers (With 100% lawyer skill) and an 100% skill Law Firm Executive to be active in the law firm. The event can happen at the end of each business day (Law Firm being active). It needs to be extremely rare, perhaps 0.1/0.01% chance, not 1% chance as if the players have money to afford a law firm they are most likely very far into the game, hundreds of days played. The player will get a notification that says “A Class Action lawsuit against [In game business name for example AJ Pederson’s] has begun!” They can click it and bring up a part of BizMan UI that shows the name of the business with the lawsuit + time remaining + The business’s street address. The time remaining can be generated, from I’d say is best 30-70 days, but you could go to 100. This is because, in real life, class action lawsuits can last years and even decades. Lastly, for more detail, the business that was sued should be shut down by the end of the lawsuit with a message in the market insider app like [AJ Pederson’s on street Example 1 Way was shut down due to a class-action lawsuit by [Player’s law firm name]
Now, as you can see, these are extremely high paying types. Setting up a law firm like this with a big office should be very expensive (Millions) and a smaller office with just traffic ticket law or something else for example should only cost around $300k. It should be time consuming, but should have some automated parts to prevent the player from getting bored spending days setting it up.
These are all my suggestions. I apologize if it is a boatload, but this game is already amazing and has unlimited potential, so I just wanted to add to it. 
Saul Goodman (Fictional Lawyer)
Edit 1: Added Modified Handtruck image
Edit 2: Added some details to class action lawsuit type