It would be good if there could be logic applied to custom fields.For instance, if there is a custom field called region and this has the options EMEA, LATM and APAC.
If a user selects the option EMEA the user should be presented with an EMEA custom field that allows the user to select a country from the EMEA region i.e. UK.
If the user selects APAC they should be presented with an APAC custom field that allows them to select a country from the APAC region i.e. India.
Without the logic users must either show all the different custom fields and train users to only select the value from the correct field or there is one field that has all the possible options which can be confusing and lead to the wrong option being selected.