webchick and I were discussing the progress of #301071: Remove profile module from core, and were wondering if "fieldable fields" would be a better direction. One of the issues is dealing with profile module categories. IMO, those categories are best expressed as taxonomy fields. If fields could contain other fields, then profile categories would be simple to build with Taxonomy.
I'm going to investigate and see how difficult this would be to implement, though I'm fairly raw at the fields API so any pointers would be appreciated :)
See:
#1792072: Merge field_collection module in Fieldable Fields killer feature in Drupal core
#501366: Add field grouping to core