I've seen this mentioned a few places, but the Aha! staff seems to keep deferring to custom tables as being the answer, which isn't the need.
When I create a custom field - let's say the customer's name or the segment impacted - I want to add it to the Idea, Master Feature, and Feature layouts as I want to:
Minimize maintenance of the set of valid values
Carry forward when one type of record is promoted, merged, converted to another without losing the data
Today, I must add that one custom field to each record type (in the above, I must add to Idea, Master Feature, and Feature), use the same internal reference key, and maintain the values as they evolve.
Both the setup and maintenance would be more efficient and less prone to error and data loss if you can create a custom field and associate it to all relevant record types - similar to how Atlassian and IBM tools manage.
Behind the scenes, with minimal disruption to your UI, you could maintain the same structure - but, update the front-end to allow maintenance in one area to limit the impact.