My gut says to me that this is one of those questions where if you have to ask, it isn't a gear problem.
When you need a wider and lens you will start to feel it. For example this shot
It was a tight space, with a Nikon D7100 and a 18-55mm lens, shot at 18mm. There are certainly technical problems with the image, but I would have loved to have been able to "pull back" further and get more of the room into the shot.
That shot is the same lens, also at 18mm on a D90. Is it a cover shot or award winner? No, but I like the colors and reflections of a dusk night at a random Burger King.
With a wider lens (not even on my radar at the time) I could have gotten a lot closer. Instead I had to walk around until things looked about like I wanted.
I think that is probably where you want to be (without seeing your actual shots). Walk around, get closer or further. Wait for the right light, zoom in and zoom out. Identify why you think your photos are dull and work on that.
Finally, it's digital. Feel free to botch some shots. Pay attention to some of the basic rules (like the rule of thirds) and then feel free to break them. You will start to "feel" the scene better for it. And finally PP.
I rarely do much post-processing. Most of it is cropping, and a quick run through DxO (A recent change for me).
This is 24mm (about the same as 18mm on DX) on a D750 that I shot wide and then cropped to make it feel wider: But it is nothing an 18mm or 24mm lens can't do!
My short answer: You probably don't need a wider lens as much as more time with the camera.