It is possible to backup droplet to an Image.
So it would be great if you could add functionality to Suspend a droplet for FREE.
The droplet would get backup up and destroyed but it would not disappear from the control panel. It would just say "Suspended". Then at any time users could Un-Suspend the droplet which would just basically build a new one from an image.
For the IP reservation - you could either choose to reserve it - and pay a small fee when the droplet is suspended, or choose to get a new one when you bring it back live (FREE).
At the moment - you can do this manually and then build a droplet from an image, but if you could add the functionality - it would mean that with a very little of effort on your part - you would give the users a fantastic feature.