If you design things to be simple and expandable, you don't NEED to keep adding features in order to give the functionality people have.
A Pencil has a certain set of features. It can write, it can erase.
It has other things it can do because of its design- It can act as a piece of wood, so it can be used to push things on the table, or used as Lincoln Logs.. But they don't need to ADD anything to support these uses.
You can decide tomorrow to use a pencil for things the developers never thought of- You can use it to trace leads, or use it to build a fort. You can use it to hold two things apart from one another, or use the eraser to hold pins..
These aren't features that needed to be added. No adjustments to the pencil are necessary to support fort-building. They're new and clever ways of using what's already there.
THAT's what you want to encourage. Find ways to create services that can be used in ways you never imagined.
APIs are a great example of that, but there's more that can be done. Make thing simple. Let people add their own uses for things and they'll do amazing things.