Balsamic vinegar does naturally have a little sweetness that comes through even more after it's been reduced. However, in many dishes, it's nice to use slightly sweetened balsamic reduction. I usually use brown sugar or honey, but there are a few more options. You can use maple syrup, honey, or agave nectar. If you wish to keep it keto but want to add a little sweetness, you can use monkfruit sweetener or another sugar substitute. Just be careful with sugar substitutes, because they tend to be sweeter, so add half the amount that's actually called for.