Expandable and 3D printable racking.
Updates at the bottom... Latest update 1/22/18
Yes, I'll make more updates and add-on accessories to this design. Please feel free to comment on what you would like to see with this.

The pictures shown is the first section I've built and me testing it with a few full boxes of filament. (No issues holding that up)
I left the corner locks long so you can see where they go. Yes, they are a tight fit.

The concept is very similar to the wire racking you get at the hardware store.
You have uprights with little grooves in them, a lock that goes into the groove and a sleeve that goes over it to lock it in place at a certain height.

Since it would be difficult to print wire racking, I broke this down into smaller trusses that interlock to form a shelf.

To start, you will need to print the following.
4 uprights - about an hour each.
4 locking rings - Couple minutes
4 corner pieces - about an hour each
2 main trusses - about an hour each
2 main trusses without the cross piece - also about an hour each
4 7-inch crossing trusses - about 30 minutes each.
You will also need the locking pin to keep the main trusses in place, I like the version you can cut since it's easier to work with.
Top Plate - Haven't tested the fit yet, but it's sized to fit on top between the corner pieces. It's a little over 7"x7". If your printer isn't big enough to print that, you can sub a piece of 1/8" wood in it's place.

Expansion pieces
4-way and 3-way corners for making it deeper, wider, etc..
Sleeves for expanding the uprights - Just put sleeve into top of upright and slide next one onto it.

I have NOT load tested this yet.. If you want to load test this, please share your results and the method you used to test it.
Yes, the boxes in the picture are full boxes of PLA and Wood PLA. (Printed with Glow in dark PLA)

This did take a little while to draw, please feel free to tip if you like it.. :)

Update 1/21/18 - Oops, I forgot to include the top plate.. Added..

Update 1/22/18 - Added a leg spacer for the bottom of the legs. This should help stabilize the bottom of the legs if you put the shelf high.

Update 1/23/18 - Added assembly view showing one way to expand it - Also uploaded a couple pictures of the one I'm starting. I used the 4-way corners so I could expand.

Update 1/26/18 - I drew up a cross support. It hooks into the lower bar of the main truss and uses a small ring similar to how the corners work. I'm testing it now, and should have the STL files loaded tomorrow. Stay tuned. - These are optional, but should help make it even more solid so you don't need to glue it.

Update 1/27/18 - Optional pieces - Added the cross supports and an assembly view showing how they attach. These work by attaching a small ring to the lower area of the leg and hooking in a cross piece that attaches to the truss on the opposite side. Depending on if you glued your assembly together and how many shelves you put into it, you may or may not need these. But,, just in case you have a wobble, this is one solution to help stabilize it.

Update 2/11/18 - Added ability to wall mount. I have not tested this yet. The "WM" files are specifically for mounting to the wall. There is a truss and special corners to make this possible. This is 100% compatible with everything else here, so it can be used as an expansion. Also, the lower rear truss mount is to be GLUED to the leg. This should be fairly stable.
Yes, you can mount a section to the wall, and stack on top of that with the other pieces. I do recommend you use anchors and use the wall mount corners for extra vertical sections.
Also in this update was a half plate if you have a printer smaller than 200x200.
I have not test printed the wall mounting hardware yet, please let me know if somethings not right.