Configurable custom data fields design

I did a very small PoC for this in the past, however, it never made to production environment.

This is slightly complicated, depending on, what level of complexity you wish to add or user is expecting.

This approach also mess things up because effectively you cannot possibly have normalization in database.

You need to look into data-types, labels, element level validation (numeric, range, etc.), domain validation (if gender is male then....), display locations, primary key - foreign key (custom field belongs to which domain entity), etc.

The whole process becomes less painful, if limitation of such feature is identified and agreed upon by all involved parties.