If you want the iPhone 6S's new features but aren't buying the new phone, you're kind of in luck: a jailbreak was released for iOS 9 this week, and some of the first hacks for it bring 3D Touch and Live Photos to all iPhones.
Of course, that's not entirely possible. 3D Touch, after all, requires a pressure-sensitive screen. But some jailbreak tweaks have found a way around that: rather than activating quick actions, peek, and pop with a forceful touch, those commands are activated by swiping up. So swiping up on an app icon presents shortcuts into the app, and swiping up on a photo previews it with a peek. The jailbreak tweak Forcy handles the app icons, while another tweak, UniversalForce, adds support inside of apps. A third tweak, Hapticle, adds vibrations for when they trigger. Naturally, these being jailbreak tweaks, you shouldn't expect them to work perfectly, but they might still serve as an effective preview of the new 3D Touch actions.
Be warned that jailbreak tweaks aren't going to work as well, if at all
Live Photos ought to work much more smoothly on older iPhones since the feature doesn't actually require new hardware. Since Live Photos are already built into iOS 9, the Live Photos Enabler tweak apparently just subverts Apple's code and makes the button to enable them available on all phones. That tweak doesn't seem to be working for everyone — some people on Reddit are reporting freezes and overheating — but others are reporting success. There still seems to be a lot of features to add, however, including the ability to easily preview the Live Photos without sending them to someone in Messages.
In both cases, you're clearly going to be getting a worse experience than the iPhone 6S will actually give you, but that hacky vibe — not to mention the ability to add features to a device you already own — is kind of part of the appeal for anyone eager to do this. If you want to get started, you'll have to check out the Pangu jailbreak.
A preview of Forcy:
A preview of UniversalForce: