Currently, to resize a partition with udisks one must remove and re-create it. However, recreating the partition clears all the filesystem signatures, thus losing access to any data the user could have stored there.
A good partitioning library must offer the possibility to resize a partition while maintaining its filesystem, of course when the new size is compatible with this request. My proposal is to add a service to "safely" resize partitions, and maybe also a service to query of how much the partition's size can be decreased without touching the data.
To realize the necessity of this, simply think of the common scenario when a user wishes to resize a Windows partition in order to have space for a new one to install Linux on, without losing Windows in the process.