I'd like to be able to do things like configure a URL to a web service to retrieve a list of options for the field or us the configuration to tie mutiple custom fields together.

For example:
I have a simple custom field for subtasks that displays a read only copy of a field in the parent issue. When the new custom field is instantiated I need to set which field in the parent to map to. So I'd like to be able to either pick a drop down of existing custom fields or simply provide a textbox where the name of the field to map to would be typed when the Custom Field is created.

We definitely want to implement this but we have already committed ourselves for features for 3.5 and 3.6 (so it wont be beofre then).
It will be useful thing to have.
At the moment, you can specify params in the xml descriptors that your plugin can read. I know this isn't nearly as flexible as to what you need but it is still better than hard-coding them.

Another custom-fields that is a good example of this is a User Field that that is restricted to a selected Group

Cheers,
Nick

For example, I might have a "Security Risk Assessment" value that is calculated as a weighted average from fields for "Sensitivity Level of Data", "Internal or External Data", "Value of the data".

Perhaps this wouldn't be a "Configurable" field per se, maybe it would be a custom field type – not sure if this is something I could build.

http://www.atlassian.com/software/jira/docs/api/latest/com/atlassian/jira/issue/customfields/impl/CalculatedCFType.html

So I guess my suggestion might be that it would cool if there was some configurable customization that could enable end users to quickly and easily leverage other field values.