40 crew and a 1500ton Freighter is a good size

, equivalent to a medium sized business. Here's some random ideas to fill living space, no doubt some are better than others.
A commercial ship that size might have child care facilities and "family size" accommodation. Star Trek style. Which could lead to educational facilities for the crew and perhaps a stateroom for the nanny/teacher (paid for by the family of course).
I would also consider a separate Officers gallery, more spacious than the crew gallery.
A small movie theatre, gym and a games room for the crews recreational time, ideally two or three games rooms so the nearly 30 people off watch at any one time have got room to spread out and avoid those they conflict with.
A board room large enough to comfortably seat the Captain and say a third to half the crew.
As a very large Far Trader (I read speculator, but that might not be what you mean), I would add a sumptuous office, designed to impress the locals when making deals. Tied into that perhaps a "samples" room with examples of exotic products that can be sourced in volume by the Trader, or perhaps what is for sale today.
An office for the 2IC, whom can be expected to be kept fairly busy with Human Resource issues (or should that be Sophont resources?).
A secure comms station, might be in order to enable the Captain to get up-to-date prices etc as soon as the Trader enters the system. Also doubles as secure "Captains-eyes" only comms for dodgy deals.
A high tech sick bay with a suitably qualified doctor is guaranteed to win respect on many worlds. Everyone has a relative that could use advanced medicine and that can help win contracts.
A small store selling over the counter drugs, junk food and personal hygene products. Might be manned by the laundry crew.
Depending on the Captain's interests, a simulator. For example laser weapons, missiles or small boat.
A fully kitted out Engineering workshop. From Lathes to high tech welders and a large stock of material to form the basis of any new fabrication. Ditto to an Electronics workshop.
A very large, secure cupboard (or 3) for crew to store personal effects they don't want in their staterooms.
A safe room, for VIP's to hide in if the vessel is attacked.
A large formal entry hall off the main airlock, like the Captains office, designed to impress. Perhaps with displays related to the ships exploits.
Multiple air locks, one main, numerous secondary.
Emergency alcoves. Providing food and life support for individual crew in emergency.
Dedicated rooms for trades training, especially engineering, perhaps maintenance and electrical as well.
Hopefully food for thought

This is definately not exhaustive.
Cheers