You could try to pick up and knit more garter all the way around the bottom. I would try that first. I've done it adding a ribbing to the bottom of a sweater that had no border and you'd never know there was an amendment.
The good thing about adding the garter is that if you don't like the way it looks you can just take it out without having done anything to the sweater first.
If possible, you could add the additional border in another color so it there is a noticeable difference, it can look like a design feature. You could also add some of the different color for a couple of rows to the sleeves to carry it through. I'm sorry, I'm getting carried away here!