In addition to the above suggestions, encoder and other sensor mounting brackets are another common use.
Also, remember that objects that are 3D printed via the FDM process (where ABS/PLC plastic spool is melted and extruded into the printed object) are weak in between the vertical layers. There are two ways to deal with this: 1) accept this and design/layout the parts with this in mind; or 2) design hybrid parts that use other parts or hardware to reinforce the 3D printed parts.
One final note: FDM process 3D printing is generally pretty slow. It might take 12-24 hours to print some parts. Some parts may actually be much faster to make via drill press/band saw/lathe/mill or purchase as COTS parts. Make sure you balance your resources. You don't want to design so many 3D printed parts that it takes 7 weeks to print them all.
