The rumors of Force Touch on iOS exist as long as the Watch exists. And since Apple added a similar technology to the Mac recently, it's only a matter of time before this tech appears on iOS. But adding the feature is one thing. Implementing software features that accept the input and use it is something else. You have to take existing devices in account (apps still need to support input for non-force-touch devices), accessibility (not anyone can do this easily) and discoverability is another big factor.
Personally a see four ways Apple could implement this:
Option 1: Replace existing touch events or buttons.
Touch events on iOS are often overloaded and complicated. Force Touch could take an existing way of doing something and make it easier and faster.
Similar to how the universal swipe back replaced back buttons and made navigating on big screens faster. Or how pull to refresh made reloading content simple and intuitive.
I see Force Touch replacing the "long press to access options" feature that now exists in many apps. Most of the time it's used to show the sharing sheet in some variation, so force press to share could become a thing.
Eg: force press a tweet in Twitteriffic to retweet or share. Or an article in Unread to save it to Pocket. Or the Springboard to rearrange or delete apps.
Option 2: System wide gesture
Buttons are almost always used for system wide actions. Volume up, mute, go to the home screen,... The home button specifically is loaded with a lot of stuff, but most importantly: long pressing it loads Siri. If you see the screen as a big button that only Apple can access things become interesting.
Swipe up brings you control center, down gives you Notifications and the Five Fingers Death Pinch returns you to the home screen.
Apple can use Force Touch to open the new Proactive Spotlight from anywhere in iOS. Could be useful if they want to make the feature more prominently available. The downside: using the entire screen and all the new tech behind it to load one app seems like a waste of creativity and possibilities.
But when you consider the Friends button on the Watch, there's precedent for something like this.
Option 3: Expanding UI
Force Touch could also be used as a new feature for apps. But there are a few issues with this:
Where on watchOS feature is used to show what would be in navigation or toolbars on the iPhone, here it would be an additional place to put stuff.
There's a risk the 'hidden' Force Touch menu could become a junk drawer similar to the hamburger menu's that were used by Facebook & co a while back. They were often a cluttered list of 'best of the rest' options that where not useful to display in the main navigation.
If any developer can add hidden options behind their app, discoverability becomes a major issue. There's no indication what can be force pressed and what not. You're never sure if something should happen if you should press longer or if it's just a regular button.
Without specific guidance this would become a Wild West of creative experiments that would only confuse or frustrate users.
In a way option 1 above is a subset of this third option. But at least there we have a precedent.
Option 4: Unpredictability
Apple could add features to iOS we don't know yet specific for this option. No one expected Touch ID and especially not the integration with passcodes, Apple Pay,...
Linking Force Touch to Spotlight is one way it could be linked. But triggering a universal dark mode, open an in-app search mode, quickly displaying the settings for an app,... Are just a few ways Apple could surprise us.