To me, the next step will be printing of wax forms for gun parts, which can then be used as “cores” for casting identical metal parts through the lost wax process. This will get you inexpensive, intricate metal parts that can be made in a shop no larger than an ordinary garage.
The step beyond this is to make a new design which is optimized for printed parts. The AR15 was designed around the capabilities of various metal forming and machining techniques.
The 3-D printed gun will have arrived when it is designed to be easily manufactured by a printing process. The stock, barrel, and many parts will be conventional, but a receiver designed to be printed would
knock gun control in the US into a cocked hat.
I would say the next step is indeed making lost wax molds for the 3D printed parts, and the easiest casting metal other than pot metal would be brass. A lower AR receiver in brass would work. Mix in some other metals and you can get a harder brass. many early rifles and pistols were made from brass.
Wax specifically engineered for lost was casting is a standard 3-D printer material.