Honestly, nothing in my opinion. The 680s sli are the equivalent of a 780ti, so you're covered until next gen is released (900series etc). You have an SSD, nice cooler... You're set!
Only thing i could think of would be some more ram (to 16gb) and/or higher speed ram.
You may want to look into a z97 at some point too as they OC better than z87, but not by much. Z87s are great