The De0-nano has already proven that it works with an external board like the Mesa 7i76, but you can wire stepper drivers to the fpga pins if you want.
If you want to run on a snickerdoodle then you have to find out if that will work. I suggest you ask the question on the google group list. Because I don't know what effort that takes.
If you want ethercat then you are going into a very specific direction. That has been done, but it's certainly not standard and you should find out if the effort you spend on that is worthwile. Especially if you deviate from known to work hardware.