As you say, it's the right size for the twin bed. If that's where it'll go, it doesn't need borders.
If you want it on another size bed, it may need borders.
"Slab" borders look like you got bored and just added slabs to make it bigger, IMHO.
When I've run out of enough material to continue the pattern, I've made borders that continue the lines into the border with the fabric that gives the most POW, if there's any left. So the idea here would be to make a section that continues the bargello into the border. the colors above and below this continuation of the pattern can be the same or different. Here's another saying,"Make visual decisions, visually", I first heard this from Ruth McDowel in her class. So, lay out the quilt with border fabric options, then lay squares of the bargello fabric over the border to get the idea of how it will look.