How exciting for your niece! And how lucky she is to have a generous and thoughtful aunt like you! As you seem to hint that you are not yet 100% decided on the nature of your gift, I thought perhaps my experiences could be helpful.
Of the lovely gifts that I received when I finished law school (in my 30's as a mature age student), a couple remain useful, even after I've moved on. One was a beautiful, small, brass Tiffany clock, which perched smartly on my desk. Another was a leather portfolio for a pad of paper; and even though it fits both 8.5 x 11 inch and A4 pads but not US legal-sized pads, I've toted it to gazillions of meetings.
Where and what type of law she'll practice will have significant bearing on what accoutrements she'll want or need, and some of these will become clear once she has been doing it for a bit. While new lawyers tend to do a lot of drafting, in my experience most of it was done at a keyboard or by hand on a yellow legal pad. It does strike me that your lovely fountain pen idea could be something of a gamble if she's not already a devotee: as she'll be in a swirl (maybe even vortex) of change and intense pressure and a near-vertical learning curve, would she embrace one more new thing?
She may need a new briefcase or other EDC for work, and what sort could vary widely. In my first year of practice I invested in a gorgeous, capacious black leather Coach computer briefcase, but after a year or two it had to play second fiddle to a black nylon rolling briefcase in order to save my back. A TB briefcase might or might not fit the culture where she's going to practice.
Might your niece have to travel in her new job? If I were still practicing I could see myself using a black Tri-Star or Western Flyer--or Aeronaut, as she'd likely need a separate briefcase for working on the plane and taking to meetings/work sessions. (In my third week of lawyer work I was sent solo with one day's notice on a due diligence trip with the CEO and other top executives on a client's company plane; when my supervising partner said not to let the client know that I was so freshly minted, I raced out in a panic to buy an overnight bag and ended up, as the department store was closing, with an overpriced rolling garment bag ... only to provoke a comment from one of the executives about my firm's rates being too high if I could buy that bag. Ouch! If only TB had been in my life back then.)