-ing: This suffix indicates an ongoing action, so you could use "sagging" itself to describe the act of drooping or falling downwards.
-ed: This suffix indicates a completed action or state, so "sagged" would describe something that has already drooped or fallen downwards.
-ness: This suffix indicates a state or quality, so "sagginess" could be used to describe the condition of being saggy or drooping.
-y: This suffix is used to indicate a quality or characteristic, so "saggy" would describe something that has a drooping or loose appearance.
For example:
* The sagging roof threatened to collapse.
* The floorboards had sagged under the weight of the heavy furniture.
* The sagginess of the mattress made it uncomfortable to sleep on.
* The old sofa had become quite saggy.
Ultimately, the best suffix to use will depend on the specific meaning you want to convey.