It'll ferment out until the alcohol tolerance of the yeast is reached. It will also thin out the body some, too.
For example, if you used champagne yeast, you could probably get 18% ABV before the yeast pooped out and reached their limit. So, you'd have rocket fuel for sure!
If you want to sweeten as is, you have a couple of options. One is to stabilize the cider by adding campden and sorbate and then sweeten to taste. You will not be able to bottle carbonate that way. If you want to still carbonate this batch (and don't keg), you can add lactose, splenda, equal, etc to taste.