What are some of the more common elements of IoT designs for sensors?
In terms of how to leverage common elements, please take a look at mbed.org.
They try to do what you mention above.
A platform where people can leverage work of the community to build various hardware applications for wearable devices, smart appliances, remote motor control, etc .
You have the choice for the base platform and then you add to that the modules that you need for connectivity and the sensors.
The intent is to be able to prototype a specific application in a few days then take it to production if it turns out to be of value.
You have all sorts of add on modules with their drivers, from sensors, display and cloud connectivity.
as a bonus, you also get a web based compiler and linker !