First I'd start looking at a possible trim material to compliment the color of the brick and keep it from looking bland.
Secondly, try using slabs and stairs to flair, or concave in to doors, windows, entrances, or to smooth out edges and slopes. Depth and that smaller detail really makes a difference.
I would also expand your skylights to cover almost all of the roof, and changing the roof to more of a gray color, or even gravel. Possibly even adding functional or decorational generators on the roof as well.
I would not change the walls EG: cave stuff in. What I would do is keep the original shape of it as it stands so you don't lose space for machines.
BUT, you are going to add to the outside of it.. Stairs, upside down stairs, slabs, support pillars, ANYTHING to get it to not be a square anymore. It can be a square inside sure for machines, but that outside you need so much depth there to stop it from being flat.
Please God add depth!
That middle part needs the most work. That huge brick square.
Just by framing it some with different materials and stairs and slabs, and make it not a square anymore will help immensely.
i watch grian all the time, his tutorials have helped me build heaps of houses. the only reason i havent used his or welsknights tutorials is because they do more medieval themed builds. im looking for a more 1930's brick factory. anyways,
this is the work ive done so far
the soul glass in the upper windows is supposed to look like hella dusty windows
i wanted to make the roof a darker grey but the only grey blocks i have are all far too light and the grey marble block i have, which would have been quite suitable for the colour roof i was going for, didnt have any brick variants just the raw form its found in and a slab version. not sure how i would go about making a roof out of gravel. maybe carpenter's blocks but that would add to many tile entities(?)
i also dont really wanna add any sort of power generation to the building itself as i have my windmill farm just behind the factory and another powerplant will be getting built not far from it since it sits so close to another village of mine and my trainstation. i might add solar panels to a part of the roof at a later date though, just depends on how my power needs go after ive rebuilt my train network