Maze generating one liner
Python 3, 226 224 bytes

My first Python golf, so probably very sub-optimal.
It produces a whole lot of trailing whitespace, but there are no preceding newlines, and at most two preceding spaces.
The input needs to be given by hand from command line (maybe someone knows a shorter way to get multiline input in Python...).

e,z,s=enumerate,'0',list(iter(input,""))
p=''.join(s)*5
r=[len(p)*[' ']for _ in p]
for y,l in e(s):
 for x,c in e(l):
  for i in range(-2,3):r[2*(x+y+(s&gt;[z]))+i*(c&gt;z)][2*(x+len(s)-y)+i*(c&lt;z)]='#'
for l in r:print(''.join(l))


The idea is to initialize a huge array of spaces r, then iterate through the input and replace the spaces with # as needed, and finally print the whole array.
A trick I used is to compare characters to z = '0' instead of testing equality to '/' or '\', which saves a bunch of bytes.