I dont understand why someone else has the rights to expansions that are from someone else's IP. And yeah SW sequel would be awesome.
Generally when you set out to do an expansion for a game, the first thing you do is make sure you've got the rights to do it. Which means checking to make sure you've still got the rights or signing a licensing agreement with whomever it is that owns the rights. And chances are that at that stage you also find a distributor to finance the project.
In this case, the answer to that would almost certainly be either 3D Realms or the distributor. But you'd really have to go back and look through the records from the bankruptcy proceedings because anything of value will typically be sold off to pay any creditors that might be owed money.