Unless there is a very specific use case, most tweaks you find in Cydia aren't specific to any model of iPhone.
For instance, Activator is a tweak that allows you to assign actions to various functions of the phone. Activator works the same on a 4, a 4s, a 5 and so on just as well as it does on a 6s. On the otherhand, unless you have a 6+ or a 6s+ some of the tweaks out there for reachability probably won't do much for you.
My point in bringing this up is that if you're looking for tweaks that apply only to your 6s you will find very few. But if looking for tweaks in general that run on your 6s then your options are considerably more.
As to 3D Touch, it's early days right now. Most of the devs are focusing on getting existing tweaks working with iOS 9. Wait a while though and you'll see stuff coming. 3D Touch is something a lot of devs are going to jump on.