Like I said, for practical purposes ANY transport can fulfill this role. But I look at the ships typically used for this, and I think to myself, "That's silly. That BWT/Liner/whatever has no means of gathering cargo from another ship or loading cargo onto one."
We do have the Hercules, but according to that ship's lore, it's not supposed to be traveling, it's supposed to stay at one station and load/unload the ships that come to that station. Plus it has a tiny cargo hold. For these reasons, it's rarely used, and probably never used as a transport.
The Hercules has found a new purpose as a base construction platform, so I'm suggesting/asking for "Hercules' big brother."
Its model and stats should reflect its purpose:
Transferring both solid and liquid/gas cargo, from a ship floating in space to a station and vice versa.
Therefore it should have cargo pods and/or a large internal cargo bay, tanks for storing liquids and gas, and robotic arms. Large ones for moving cargo pods, small ones for moving individual units of cargo, and some kind of arm that has a hose on it so it looks like it could be used as a refueling ship.
It should also be large enough to stand on its own as a medium-size transport (3k-ish) so that 1) people use it, and 2) when people use it, it doesn't take 9-10 trips to unload and fill a 5k transport.
With the above features, you could use this thing for more than just coming along on Zoner trade runs to load and unload ZBTs.
Mining companies, both metal and gas, could use it to collect what they're mining and fill passing transports, or haul it off to a playerbase to be put up for sale. It could also be used as a support ship, providing fuel for ships using survey/jump/cloak modules.
Again, practically, there's nothing here that can't be done by a transport we already have. What I'm looking for is a transport that looks good doing it. :)