Plum Ugly Quilt?
 

When I began this quilt top using blocks received in a block swap years ago, I thought it would be massive ugliness (is that a word? LOL). However, since putting it together I find that I REALLY like it! Now I just don't know what to do for borders; I would like it a little bigger. Any suggestions for me?
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You have a lot of color in the quilt, so I would probably use black or a gray.

I usually go with a medium to dark blue border for for scrappy quilts.

I have read that the border color will determine how the quilt is viewed; i.e., green border = it will be thought of and referred to as a green quilt. For a scrappy, not sure this even matters to you. :o

I'd continue on out with the sashing size patches. I have one quilt I had 1" finished squares 5 deep for the borders. Great way to use up small scraps and get a Wow finish.

I would put a narrow solid border on and a piano key border last.

i would use brown

And choose the color YOU like best for the solid!

I agree with MadQuilter and quilting cat: narrow solid border of your choice follow by piano keys. If you don't want to work that hard, tho, I'd do a narrow bright solid (or small print that reads as a solid) surrounded by a larger border of a dark neutral (gray, black, navy, dark brown).

It's not exactly what you're asking, but one way to increase the size would be to splice the quilt on the diagonals & add fabric that way. By adding 2 high-contrast colors arranged in a triple strip across the diagonal lines of the quilt, it will draw most of the attention to those lines, rather than to the background you say you don't like. There are many different options for that. I played around with some different options in Paint & there were several that worked, depending on how you want the final quilt to look. Here's one example with a navy I pulled from your patchwork & white (I'd go with a snow white so it works with the rest of your piecing):
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Or if you're truly set on the name "Plum Ugly" -- you could go with a dark plum instead of the navy. :D That would also give you some lovely white space in which to do some really striking quilting & again give a new focus to the viewers' attention. If you do the X, I probably would only add a very, very narrow stop border on the outside edge, just so the beautiful scrappy piecing doesn't get covered up with the binding. I'd probably make it 1/2" wide, so that 1/4" was all that showed after binding. I'd keep it a white stop border & bind in the same color as whatever you use for the X.