e.g. I create a custom goals field for ideas, I then want to be able to create a report to show the idea with goal name and the goal success metric and time frame.
I can see how it would be really powerful to generate reports that join via custom fields.
Unfortunately it is unlikely we will be able to support this. Joining tables is computationally very expensive allowing arbitrary joins would frequently result in reports that would take too long to generate.
The joins that are currently allowed have been carefully optimized to give good performance, even with thousands of records.